Gemstone Testing tools

by Ms Nandita Ray

It is tricky to test a gemstone. Some stones are soft, some are mounted while some can be coated, treated or easily scratched. One has to be very careful so as not to damage the stone in any way. With technology advancing rapidly, the tools used for testing are made to suit ones needs and pockets. Aiding the tools is a wide range of database, which lists the particular characteristics of gems.

Tools Required to Test Gemstones

There is an absolute and definite need to test gemstones, especially if they are expensive. Competition, economics, availability and demand have all worked towards the explosion of the gem market. The markets are flooded with gemstones of various hues, brand and breed. Therefore the need to test the authenticity of gemstones is imperative. Insurance too has a hand. They need the gem to be certified.

In the past, it was pretty easy for a trained gemologist to distinguish a flawless gem from a flawed one. There were no synthetic gems during those days. Today however, one has to determine whether a gem is real or not. Trained eyes cannot always avoid error, particularly, if the gem has been manufactured in the laboratory. It becomes a challenging task. Earlier on a magnifying glass was good enough. Presently however, a host of instruments are used, especially if one wants to be absolutely certain of the gem, its history and pedigree.

It is tricky to test a gemstone. Some stones are soft, some are mounted while some can be coated, treated or easily scratched. One has to be very careful so as not to damage the stone in any way. With technology advancing rapidly, the tools used for testing are made to suit ones needs and pockets. Aiding the tools is a wide range of database, which lists the particular characteristics of gems. This is valuable because the markets are deluged with zircons or the synthetic Moissanite, which are look alike diamonds and very difficult to differentiate from a real diamond.

Loupe

Loupe is actually a sophisticated magnifying glass. It magnifies the gemstone in detail. The internal flaws, surface blemishes, cracks and any other cover ups are easily seen. It is a powerful convex lens that produces a magnified image of the gemstone. The standard loupe that used is 10X, which magnifies the gem ten times its size. This standard is mandatory for testing gemstones in the USA.

Binocular Microscope

image: www.ottofrei.com

Binocular Microscope is equipped to spread light from the bottom and from the sides. Its function is similar to the loupe. Presence of the lights and its superior optical strength provides excellent clarity. It usually has 10X magnification power though 200X can be got. Most common ones are 10X to 40X. There are stages of adjusting the iris diaphragm. It also provides coarse to fine focusing.

Refractometer

Refractometer measures the refractive index of the gemstone. This is needed in order to find out its composition and physical properties. It measures the extent to which the light that passes through the gemstone from the air is bent. It helps identify the gem by comparing the refractive index thus got with the established values. It assesses the purity of the gem by comparing it to the value of a natural gemstone. It also determines the amount of solute that has been added to the gemstone by comparing the refractive index thus obtained with the standard curve that is established. This is done by putting a drop of a liquid with a high RI, on a glass disc which is based on the tip of the Refractometer. Natural or white light is passed through this. The RI is reflected in the magnified gauge.

Dichroscope

Dichroscope is used for the quick identification of gemstones. It can effortlessly separate gemstones based on dichroism, which is the property of a gemstone to show two contrasting colors when turned in two different directions, under a light. This is used to distinguish two physically similar looking gemstones. The dichroscope is used to view the dissimilar color tones of the dichroic piece. It can easily differentiate a Ruby from a Tourmaline, which is dichroic while a Ruby is pleochroic. Some gemstones that are dichroic are Sapphire, Topaz and Zircon, to name a few.

Polariscope

Polariscope is used to determine if a gemstone is natural or not. It can differentiate diamonds from Moissanite. It also detects strains in diamonds. It can differentiate between gems that have double or single refraction i.e. birefringence. Double refraction is the breaking up of a ray of light into two rays, the ordinary ray and the extraordinary ray, when it passes through some materials like calcite also called Iceland spar. This effect can take place in gems that are anisotropic in structure i.e. having properties that differ according to the direction of measurement. The polariscope helps to track and see the path the rays of light are taking through the stone that is being tested. By measuring the optic characteristics of transparent to translucent gem material, the refractive index is found. Double refractive is only a property of solids. For transparent materials, the graph of index, unlike the wave length, is curved with a few general characteristics. A gem is held between two polarized plates and the light transmitted through it is studied and based on the readings the identification is made.

Some gemstones with double refraction are Quartz, Tourmaline, Ruby, Peridot, Sapphire, Zircon and Rulite to name a few. Diamonds are isotropic, meaning single refraction. Topaz, Mica and Perovskite have triple refraction.

Ultra Violet Light source: – some gemstones start emitting a fluorescent glow after being exposed to short or long ultra violet radiation. Many gems can be easily recognized by the Fluorescent color and the locale. Some fluorescent minerals are Agrellite, Tugtupite, dolomite, Fluorite and Benotoite.

Color Filter

Color Filter is also called a ‘Chelsea Filter’ or ‘Emerald filter’. This instrument helps in separating natural, synthetic and imitation gem material. It helps in establishing whether similar looking stones exhibiting the same colors are natural or not. It comes handy while dealing with some green, red or blue stones, like blue Spinel and blue Topaz. It detects dyes in gemstones also.

Spectroscope

Spectroscope is also used to separate natural gems from synthetic gemstones. This is used in specialized gem testing laboratories. The differences in the chemical composition are revealed by the absorption spectrum of the light transmitted through the gemstone that is being tested. The instrument breaks up the light that is being transmitted from a gemstone into its spectral colors. This helps in testing the various wavelengths that are being taken in i.e. being absorbed by the gemstone. This indicates the type of elements that are present in the stone, which are absorbing the wavelengths. It allows the gemologist to see which color of light is being absorbed and thus identify the gemstone.

Specific gravity Liquids

Specific gravity Liquids – this is important for identifying a number of gemstones. A carat scale also called hydrostatic balance is used.

X-Rays

X-Rays are sometimes conducted to separate natural pearls from cultured pearls.

Diamond Tester

Diamond Tester is used for testing diamonds. The unit is switched on and the diamond that is to be tested is touched by a pointed tip. A light will come on to indicate if the stone is a genuine diamond or not. It uses thermal conductivity to differentiate diamond from its many stimulants. When the tip is touched to the diamond, the meter will show green if it is a real diamond if not then it will remain in the red zone. This instrument can be used to test the minutest diamond, as small as 0.02 carats. It can also separate colored stones from one another. By merely touching the thermo electric probe to the gemstone, the relative heat conductivity of the material is exhibited on the dial.

These are the basic instruments. There are improved and sophisticated versions of these instruments available in the markets.

Gemstone Cutting

by Sheweta Dhanuka

Gem cutting is a long process that takes many months to convert simple rough into fascinating gemstone. It involves many steps. In order to acquaint the reader with the overall process of gem cutting, all the steps are explained one after the other. The steps elucidated are not in any fixed sequence.

Gemstones in the jewelry items enhance beauty of the women making her more attractive and splendid. Gem makes women beautiful and gem cutter makes gems beautiful. When woman is seen wearing the gemstone jewelries it is in it’s finest form but initially as rough any precious or semi precious stone is as simple and pale as a normal pebble. It achieves it magnificent look after it undergoes the gemstone processing. This process is known as Gem Cutting or Lapidary that involves cutting and polishing. An individual performing this art on the stone is known as gem cutter or lapidarist.

These amazing gemstones are created deep under the earth crust by the grace of Mother Nature. Innumerable chemical reactions take place in varied weather conditions like high temperature, pressure etc and then stones are retrieved after difficult mining. The material procured as the result of mining is referred as rough. The rough undergoes gem-cutting process in order to get transformed into small stones so that it becomes wearable and useful for other purpose.

Gem cutting is a long process that takes many months to convert simple rough into fascinating gemstone. It involves many steps. In order to acquaint the reader with the overall process of gem cutting, all the steps are explained one after the other. The steps elucidated are not in any fixed sequence. The reason being, it is not necessary that each single stone would go through all the mentioned steps like for example precious stone diamond is hardly ever seen in form of beads so for this gem drilling step makes no difference. Similarly, for gemstone comprising infinitesimal inclusions, two almost same steps like grinding, sanding gets clubbed into one.

Gem cutting process involves lapidary techniques that convert bleak rough into eye-catching gemstone. The well-defined gems then are used in distinctive ways viz. cabbing, faceting and carving. Be an adjunct with the above-mentioned techniques, the below text also mentions about the different shapes and mixing styles practiced in the gemstone industry.

Lapidary Techniques

These are the first and foremost techniques of this crucial process where rough is lightly hammered to confiscate unwanted part like brittle and fractured area of the stone. It is performed in eight different stages starting from sawing, grinding, sanding, lapping, polishing, drilling and tumbling.

Sawing

As the name specifies, in sawing a saw or cutter is used to convert rough into small stones. The saw is sharp circular blade made up of steel, copper or phosphor bronze alloy and its surface periphery comprises diamond clench. It is infused in the rough and then whirled at many thousand feet for every minute to make the gemstone. After heating in order to remove unwanted material from the stone, it is washed with either with water or oil. Washing helps stone and saw from getting overheated. This entire process requires lots of concentration and expertise, one single mistake can result in severe damage. Therefore, intense precautions are taken while sawing. It is brutal to hold the rough with naked hands while sawing; consequently the person covers hands and face with appropriate safety gadgets.

There are mainly two types of saws viz. circular and jig. According to the rough and the gemstones to be retrieved the saws are selected. Saws are available in different sizes.

Circular Saws

These are the most common saws used by gem cutters as it is suitable for the majority of roughs. The edge of the saw is plane and sharp. There are three different types of circular saws.

Slab Saw

Slab saw consists of 16-24 inches diameter. It is used for cutting thick rough into thin slab stones.

Trim Saw

Trim saw possesses 6-10 inches of diameter. It is used for cutting smaller roughs into small stones.

Faceter’s Trim Saw

This saw has minimum diameter that is of 4 inches, least amongst all. It is used for cutting the most valuable rough into small stones.

Jig Saws

As the name specifies, the edge of the saw is jig jag that is helpful in cutting curved lines in the rough. It is used along with a thin metal band or wire. This saw minimizes the wastage therefore is mostly used for costly stones like Ruby, Emerald, Alexandrite and so on.

Grinding

Once the rough is sawed into small stones then after each single stone is rubbed against either diamond – infused or silicon carbide wheels to bestow the preferred outward appearance. This is referred as preform. At this stage many times the surface of the stone is left with undesirable scratches. Subsequent to grinding the unwanted grits and mesh are removed by cleaning the preform with water or oil. It also acts as coolant as it averts the stones from excessive heat.

Sanding

In this stage the stone gets the flat and smooth surface. Through the use of excellent abrasives, the graze and scratches are removed from the stones that are left behind in the previous step. It is the crucial step for shaping the stone as from now the perfection in final appearance begins.

Lapping

This step of gem cutting process is quite similar to previous two steps that is grinding and sanding. The stones are stroked against one side of either gyratory or vibrating flat disk in order to give the flat surface. The disk is made up steel, cast iron and copper bronze alloy etc. It is known as Lap.

Polishing

It is the most important step as after this step the gemstones attain the mirror like appearance that helps in light reflection from its surface area. Different types of polishing agents are used for defining the overall look of the stone. Alumina, cerium oxide, tin oxide, lead, ferric oxide, chromium oxide and silicon oxide etc are few names of the polishing agents used by the gem cutters. Ferric oxide also known as jeweler’s rouge and silicon oxide called as tripoli are mostly used. According to the surface of the stone the polishing agent is selected for example flat surface stones are polished with tin oxide, lead, pellon or leather. For round surface felt, leather, cork or cloth is used.

Drilling

As the name specifies, in this step a small hole is created through the stone, such stones are known as beads. Only some semi precious stones are used for making the beads as precious stone and other semi precious stones like alexandrite etc are too expensive. Small revolving rod or else tube with a diamond or silicon carbide tip is inserted to drill the hole in the stone. Silicon Carbide consists of 9.5 Mohs while diamond is 10 Mohs on the hardness scale. The former is the synthetic compound of silicon and carbon. This manmade amalgam is used mostly for softer gemstones. Ultrasonic or vibrating drills are the most effective source for drilling the gems but are used exclusively for huge amount of beads, as it turns too costly for making small number.

Tumbling

It is the mode of giving the desirable attractive shape to the stone that are formed roughly in the lap of natural environment. Large numbers of unappealing shaped stone are turned in a tumbling barrel at a dawdling speed along with abrasive silicon carbide and water for days together. Stones are rinsed vigilantly between the grades there by attaining the smooth and polished look. There are two ways for tumbling the gemstones. One is rotating while other is vibrating.

Rotator Tumbler

These are the barrels hexagonal in shape that aids in the rotation of the gems. In rotator tumblers the gems are turned fully upside down. In order to know the progress of the gems inside the tumbler, it has to be brought to a standstill in regular time duration. Rotating tumbler is standard tumbling method and is cost effective too.

Vibratory Tumbler

These barrels are like a container in which the gems are in the stationary position but are vibrated from all sides. It is quite easy to monitor the progress of the gems inside this tumbler as compared to the rotator tumbler.

Faceting

The aim of faceting is to reveal the vividness and luminosity of a gem. It refers to the light refraction in the stone that is when light ray enters from one of the facet of the stone it is reflected back for the bottom facet. The viewer in rainbow colours views this light refraction. Proper faceting is very important for the transparent stones such as diamond, ruby, peridot, sapphire, topaz and many others. Well-created facets in the stone amplify its net value magnificently.

An individual creating the facet is known as facetor. He/she cuts and polishes the facet on the intact surface of stone using the faceting machine. The machine consists of a motor that holds the hand piece assembly by rotating the lap, water supply and adaptable hand piece along with index gears, protractor and mast/platform. There are two types of faceting machines – one is left that is known as mast- type and other is right faceting machine. The later is further having two varieties viz. hand piece and platform.

The flat surface of the gem is known as facet. Each facet on the stone is created in a proportioned pattern. First the facetor immerses the gemstone in the adhesive wax, epoxy or glue and sticks it on a metal dipstick. Later it is placed in the hand piece to get the position control. Then after cutting angle is adjusted vertically with the help of the protractor while rationally it is done through index gear. Finally then facets are created after grinding, sanding and polishing on the revolving lap. In the regular intervals water and other solutions are used as coolant and lubricant respectively. This cycle completes one side of stone. A jig is used to change the side on the dipstick. In the same way by repeating the steps opposite side is finished too. Normally flat facets are seen but with modern technology and innovative ideas of facetors, new-fangled concave facets, grooves and much such novel stuff are created.

Cabbing

It is also known as cabochon cutting, the simplest and most common lapidary form. Opaque and translucent stones are designed in this variety. Transparent stones with too many inclusions are also cut into cabochons to use it optimally. Cabochon at top are smoothly curved and polished while on bottom it is flat or round that could be polished or sanded as per the jeweler’s requirement. Turquoise, opal, bloodstone, lapis lazuli etc are mostly shaped into cabochon.

Cabbing procedure is almost same as faceting. Cabochons or cabs are cut through the process of dopping where the stone is attached to the dipstick with the help of either adhesive wax or glue. Later the stick along with the stone is rotated during all the lapidary techniques to have the smooth rounded surface all over the stone. Dopstick is made up of metal or wood. Separate cabbing machines are also available. These machines consist of quite a lot of wheels having diamond or silicon carbide gravel, twirled by one single arbor and motor. It also has a continuous water supply that acts as a coolant/lubricant to rinse off unwanted fragments and protects the stone from excessive heat.

Carving

It is a creative art in born in the carving artist. It is the most difficult art as the craftsmanship involves deep concentration, patience and imaginative skills. Apart from these talents he/she must also possess methodical knowledge about the lapidary techniques then only they could give astonishing look to their respective artwork. One who carves the costly gems is a skilled individual, turning the plain stone into captivating masterpiece having the functional or decorative use.

Semi precious stones are suitable for carving as precious stones are extremely valuable and pricy. There are many different types of carvings done on the gemstones. Some of the most common ones are :

Cameos

Stones and sea shells are referred as cameos. It is the carved portraits in any size of the stone. The portrait is carved above the surface of the gemstone. It seems striking because of the natural colored layers and remarkable carving. The wonderful blend of God’s creation and human art in one single frame looks amazing. Semi precious stone Agate is most commonly used for making this carving item however; some fine artist also brings other stones into play. Italy and Germany are the main source of premium cameos carved from shell and agate stone respectively across the globe.

Intaglios

This art conception is quite similar to that of cameos. It is carved on gemstones and seashells as well. The difference between the two is of carved portraits. Intaglios are the portraits that are carved down into the surface of the stone while the portraits are raised above the stone surface in cameos. The beauty of the carving emerges due the different coloured layers displayed in the stones through the natural formation. Italy and Germany again are the preeminent foundation of eye-catching art pieces of this form of carved item.

Sculpture

These are the carved forms of living things, it may be any animal, bird or even the human face or body. This is solely created and procured to decorate unusual corners of the dream home. It is mentioned especially for dream home because sculptures are relatively classy and pricey when compared with other carved forms.

It is priced high due to three reasons. First, creating a sculpture requires extreme expertise and artistic skills, as all the organs of the living creature should be well defined. There is no scope for minute errors or slight change else the final look of the art piece differs from the look of the living being. Artists take months to do fine detailing of their masterwork. Second reason is the cost of the raw material. The main raw material for carving sculpture is the stone. The price of the stone itself is very high. Further, the cost increases with the increase in the size of the stone. Moreover, for creating any size of carving the stone turns out to be more costly than the wood or clay. If it is carved on the single stone then the price is increased more. Finally the third reason, additional use of other coloured stones or diamonds to give it more natural look makes the price touch the sky.

Stand Alone Carving

These are the carving pieces that are not used for making jewelry items. They are created for decorative purpose or for some practical use such as ash trays, jewelry boxes, pen stands, key chains, paper weights and so on. These fine-looking objects are always in demand by the gemstone lovers.

Beads

These are engraved as small spheres first and later drilled to make beads that could be string together in a string to make the diverse jewelry items like necklace, earrings and hand bracelets etc. In order to make spheres, the lapidarist saws cubes or dodecahedrons from the rough. The cubes then are shaped into round by grounding it between the rotating concave cutters. The stone revolves around the concave cutter irrespective of the direction and attains the desired spherical shape. The spheres formed, then undergo the lapidary process of grinding, sanding and polishing. Finally, these are drilled to form the beads.

Once the beads are formed, it is taken over to bead mills for grinding and sanding in order to make high quantity commercial beads together at the same time. As a final point, these are polished through tumbling. Bead mill consists of two laps viz. one grooved while other flat. The spheres turned into beads are rolled between these laps to obtain the perfect pet shape.

Mixing Styles

The universe is crammed in with immense talent. People across the world use their groundbreaking ideas in each and every field like computer, fashion and so on. Similarly, gemstone industry too has billions of artisans who create attention-grabbing art pieces using their pioneer thoughts be it for jewelry items, functional items or decorative pieces. Artists mix different coloured stones in one single art work in a distinctive technique. Few such inimitable examples of mixing styles are inlays, mosaics and intarsia.

Inlays

Inlays are created by mixing gemstones with materials like metal, wood and other stones. The gemstone is cut in the favored shape, glued with the adhesive wax and then fixed into a void nook of other materials as mentioned. This style only uses opaque stones as the beauty of these lies in its façade that is only the outer appearance visible to the viewer. Semi precious stones commonly used as black onyx, turquoise, jade and so on. Transparent stone requires some ray of light that refracts along different facets, sparkles and looks fabulous. Therefore, transparent stones are not suitable as in an inlay once the stone is fixed it has no scope of any light rays.

Intarsia and Mosaics

This is the unique style of mixing that involves innovative and creative designing skills. Both these art pieces are crafted by joining together the coloured stones small in size like pebbles on a flat base material like stone, wood and metal etc. All the stones are top cut and polished in a way that it looks like a picture. It could also be set in some appealing pattern that grabs the attention of the viewer at the first glance.

Then main difference between these two at forms is same as that of cameos and intaglios. Mosaic is produced on the top of plane base of stone whereas intarsia is fashioned into the surface of the foundation material viz. stone, wood or metal. The later is also called as Florentine mosaic or Pietre Dure. The name Pietre Dure suggests, the finest pattern of this skilled artwork is brought into being from the land of artists called as Italy.

Shapes

As cited, in the ingenious world both the artist and the end user are passionate about new and unusual designs. The regular shapes like round and square still exist and shall subsists forever but the demand for something new is increasing at a very rapid pace. Thus, everyday new shapes are created as per the demand of the coloured stones enthusiasts.

Round

This is the standard shape quite common in faceted stones as well as cabochons. It is mostly used as a center stone in necklace or ring fingers. The beauty of this shape increases with the increase in size of the stone.

Oval

Oval is the most universal shape in the jewelry items as it compliments all other shapes very well. It adds an exclusive look to the designs. In cabochons, this shape is idyllic and the exquisiteness of the stone is highlighted very well. All opaque stones get very well cut in this shape. It goes well even for faceted stones.

Square

It is yet another regular shape but less common than the previous two. It is suitable only for those designs that are created with respect to the square cut gems. This shape does not match well with other shapes rather has its own sole significance.

Rectangle

This shape is little unusual for regular jewelry designs, same as square. However, it could be seen in cabochons as well as in faceted stones. Big size single stone cut into rectangle shape hung, as a pendant in a thick black or multi-coloured thread looks quite trendy and is ideal for the casual wear.

Emerald

It is initially cut as rectangle then the four corners are sliced away to give an emerald cut. This is relatively a new shape being like by young generation. Precious stone Emerald in this shape is in style amongst the elite families of the society. The attractiveness of this shape too increases with the increase in the size of the stone.

Marquis

It is a new creative shape, thin at the edges and thick in between of the stone body. The top and bottom of the stone are curved pointed points. Transparent stones are more suitable for this shape than the opaque gems. Female finger rings with gemstones cut in marquis shape are gaining popularity in every corner of the globe. It is also referred as Navettes.

Shield

This shape is proportioned at the sides of the stone but the ends are cut in diverse directions. It is available in different magnitude and varied sides. This is relatively less known and atypical shape.

Cushion

It is shaped similar to the cushions of the sofa or divan in the drawing room. It is more or less like either square or rectangle with its sides smoothly curved. This shape is more used for transparent stones.

Tear Drop

This shape is like a drop of water or tear thus named so. It is pointed at the top and curved as semi circle at the bottom. This is a bizarre shape therefore need to be clubbed along with other shapes to intact the design for any jewelry item. It is also called as pear shape.

Geometrical Shapes

Geometrical design jewelries are new-fangled choice of selection amongst the younger generation who seek fun and frolic in their trendy style. These designs are created using all different shapes of the geometry like triangle, hexagon, octagon, rhomboids, parallelograms and many more. The most remarkable part of these deigns is that it makes fabulous pendants for males. Normally, females dominate jewelries but as for geometrical jewelries it is mostly for males.

Fancy Shapes

image:GemsDB.com

These shapes are fashioned into jewelries that define the sophisticated and elegant aspect of women. The shapes are customized as per the requirement of the user. Few to name are leaf cut shape, princess cut, animals, alphabets, symbols and so on. Leaf cut shape is actually the shape of a new leaf of any tree; gold or silver sleek wires highlight the veins in the leaf. This cut is mostly made in light coloured stone such as peridot, amazonite etc. The absolute gaze of the shape is very charismatic and it represents the skilled craftsmanship of the artist. Similarly, the alphabets are shaped in different varieties of calligraphy. The fancy shapes has indefinite boundaries, it all dependence on ones ideas and thoughts.

This is long journey that the gemstone covers from being found in the lap of Mother Nature to the end possessor. The entire process is intricate and requires attention to the minute details. The hard work and creativity invested gives birth to the beauty.

More Resources
Step by step Gem cutting guide
Gem cutting company
Gemstone cutting terms

Gemstone Hardness Chart

by Mark

Gemstone Hardness Chart in Mohs scale

See Also: Mohs Scale

Chelsea Filter

by Ritika Changrani

Also known as Emerald filter, Chelsea filter is used to identify natural, synthetic and imitated gemstones. Developed by the British Gemmologists Anderson and Payne, this filter was initially developed only for the separation of natural emerald from green beryl. Now it is being used to identify various gemstones.

Chelsea Filter – Gemstone Testing Tool

Chelsea filters are designed to absorb all lights except red and yellow green which are transmitted through the filter. This causes different gemstones to exhibit different colors when observed through the Chelsea filter. Emeralds, when seen through the filter, appear red or pink in colour, while ruby appears brilliant red. Synthetic blue spinals appear yellowish orange or pink while the gems which are imitations appear green or greenish grey. Cobalt glass imitations of sapphire appear deep red.

How to use a Chelsea Filter

Hold the Chelsea filter close to the eye – may be at a distance of an inch or two from your eye. Provide strong light to the gemstone by lighting with a torch or bulb. Hold the torch or bulb such that the light falls directly down on the stone. When viewed from the filter, the stone may appear to change its colour. Never hold the Chelsea filter close to the stone since it may produce wrong results.

How does a Chelsea Filter Work?

Chelsea Filter works based on the characteristics of the gemstone. The Chelsea filter detects the presence of chromium and cobalt in the gemstone. Chromium, if present in the gemstone, produces red and green colour as in ruby and emerald, and the presence of cobalt gives a blue colour. It is the amount of chromium or cobalt present which gives a dark or light colour.

When white light is passed through a gemstone, it absorbs colours of some wavelength and lets other colours free. The other colours which are not absorbed by the Chelsea filter mix up and give the colour appearance to the eyes. Depending on the appearance of the colour when viewed through the Chelsea filter, it is easy to detect the type of gemstone it is. For example, when white light is passed through the Emerald, emerald absorbs all the yellow-green wavelengths. Chelsea colour filter can filter all the colours except yellow-green and deep red wavelengths. Since yellow-green wavelength is already absorbed, only the red colour passes through the Chelsea filter.

One has to make sure that there is enough light on the stone, else the reaction may not be visible to the eye.

Gemstones identified by Chelsea Filter

Red Stones

Red stones that contain chromium are Burma ruby, synthetic ruby and red spinel. Ruby and spinel when viewed through the Chelsea filter appear red. Demantoid Garnet when examined through the Chelsea filter in presence of light appears pinkish. Though Thai ruby and pyrope garnet contain chromium, there is no colour change observed due to the presence of iron in those stones, which prevents colour change. Red stones like garnets and paste do not contain chromium and hence there is no colour change. When viewed through the Chelsea filter, they appear dark red or black in colour.

Therefore when you are examining a group of red stones, you can easily separate different kinds of gemstones. Those which impart brilliant red are synthetic rubies. Stones which glow slightly red are natural rubies and spinals. Thai and Demantoid garnet may turn pinkish or slightly red. The stones which remain dark red or black are paste and other garnets.

Green Stones

Beryl is a green coloured stone which is coloured by chromium. The different types of beryl are emerald and aquamarine. Natural emeralds have a smaller amount of chromium, while synthetic emeralds have larger amount. Hence the change in natural emeralds is to dull or a little bright red. Synthetic emeralds change their colour to bright red. Some synthetic emeralds produce a dull red colour due to the presence of iron. Some natural emeralds like South African and Indian emeralds do not produce any red glow when viewed through the Chelsea filter. Chelsea filter can just give an idea that the gemstone may or may not be an emerald but the test is not a proof of detecting an emerald.

Some stones can be confused with the emerald. They are Demantoid garnet, green garnets or savolite and tourmaline. All these stones when tested using the Chelsea filter, appear to glow red and are often mistaken to be emeralds.

Green gemstones can be distinguished from each other by some of their characteristics. Emeralds produce an oily luster when seen through a Chelsea filter. Green garnets appear brighter than emeralds. Green zircon appears pinkish in colour and exhibits double refraction. Tourmaline is also a stone which exhibits double refraction. When green zircon and tourmaline are viewed through a 10X lens, the back faces of the stones appear to be doubled. Aquamarine has iron content due to which the colour produced is dull. The colour change in aquamarine is from blue-green to muddy grey-green. Green garnet, green zircon and chalcedony show very little colour change and appear pink or red. Pastes, green sapphire and peridot appear remain dark-green. Alexandrite appears red when seen through the Chelsea filter.

Blue stones

Blue stones contain cobalt which can be detected by the Chelsea filter. The colour change of blue stones when viewed though the Chelsea filter is red in highly doped stones and orange brown or green in light blue stones. The colour change to red when viewed though the Chelsea filter is an indication that the gem stone is synthetic. Sapphires show no colour change and remain dark blue or black.

How to take Care of a Chelsea filter

• It is recommended not to touch the Chelsea filter as it may leave marks on the glass. A soft dry cloth must be used to remove the dust.
• Never put the Chelsea filter in water or any liquid.
• The Chelsea filter must not be allowed to get heated either in direct sunlight or on a radiator.
• When not in use, the Chelsea filter must be kept closed.

Chelsea filter is an important tool in the field of gemology which can be used to test various stones. Although, the test is not cent percent reliable since the composition of every gemstone is not the same even in the similar type of stones, it is used by a number of gem testers for testing the gemstones.

More Info: Gem Testing Tools

Dichroscope

by Ritika Changrani

Dichroscope is a tool used to differentiate similar looking gemstones based on the dichroic property of the gemstones.

Some gemstones are singly refractive. They exhibit only one colour to observer. There are also gemstones which exhibit two different colours of light. But most of that is not detected by the human eye since the two colours produced are so close in wavelength that it becomes difficult for the human eye to identify them. The two colours in such gemstones mix up to give a slightly different colour. Such stones are called doubly refractive gemstones. For example, ruby is made up of dark red and light pink colour. But it appears to the viewer as light red.

The property of the gemstone to exhibit two different colours or shades when turned in two different directions under light is called as dichroism. Examples of the gemstones which are Dichroic are sapphire, zircon, tourmaline and topaz.

Dichroscope is a tool used to differentiate similar looking gemstones based on the Dichroic property of the gemstones. Hence it can easily differentiate gemstones like ruby from tourmaline since ruby is Dichroic and tourmaline is pleochroic. Dichroscope also helps to determine Pleochroism and trichorism. Dichroscope helps to determine the optical properties of a gemstone. (image courtesy: www.faceters.com)

Dichroscope is a metal tube which is open on one end and has a lens at the other. It has an optical calcite mounted inside the tube and produces a double image of the square opening. Enough bright light is directed on a gemstone and viewed through the dichroscope. Dichroscope separates the different colours of light into its constituent colours. Light entering the dichroscope is broken into two polarized rays. Two images of different colours indicate that the vibrations are of different wavelengths and at right angles to each other. If dichroism is not detected in the first test, the gemstone has to be turned and output colours must be viewed in other directions too.

Pleochroism can be detected by rotating the instrument at an angle of 90 degrees. If the two colours switch sides on the split image, the gemstone is pleochroic. To determine trichorism, it is necessary to change the orientation of the stone. If one new colour is determined along with one of the colours of previous orientation, the gemstone exhibits trichorism. Trichroic gems show three different colours when viewed in three directions perpendicular to each other. Singly refractive stones produce only one colour on the dichroscope.

How does a Dichroscope work?

Dichroscope works on the principle of plane polarized light. Plane polarized light is the light of a specific wavelength that is separated from ordinary light and travels in a specific direction. Some waves may travel up and down and some side to side. When light traveling in different direction is being separated, it is possible to view the individual colours of specific wavelength.

Types of Dichroscopes

There are two different types of dichroscopes. They are London dichroscope and Calcite dichroscope.

London dichroscope

In a London dichroscope, two pieces of Polaroid orientated at right angles to each other are placed such that the light entering each polarizing filter are at right angles to each other. The two polaroids in a London dichroscope are joined together in same plane. London dichroscopes are much cheaper when compared to the calcite dichroscope but does not give as clear a result as the other. It is difficult to determine gemstones with weak Pleochroism with a London dichroscope.

Calcite Dichroscope

The calcite dichroscope is made up of a calcite crystal. The calcite has a special characteristic called bi-refringent. This property of calcite enables the gemstone to appear doubled when looking through it. The calcite crystal splits the light entering the dichroscope into two rays and thereby helps identify the difference in colours. The two rays form two images at right angles to each other.

Out of the two dichroscopes, the calcite dichroscope is the most commonly used in the gem testing laboratories to test the Dichroic property of the gem stones.

How to use a Dichroscope?

Hold the upper portion of the dichroscope near the eye – may be an inch from your eye. Place the gemstone near the opening. The other end of the dichroscope is hexagonal and hence it is easier to turn the dichroscope while testing. View the gemstone through the gem view opening. Slowly rotate the dichroscope to complete one full rotation and observe the colour difference of the two small squares which appear inside the dichroscope while you rotate. If you observe two colours while you rotate, the gemstone is dichroic. If three colours are observed, the stone is trichroic. Repeat the observation to confirm the number of colours observed. Observe the strength of the colours as it helps determine the type of gemstone. Always make sure that sufficient bright light is focused on the gemstone while you observe the colours using dichroscope. Never use fluorescent light since it can give a false result.

Advantages

• Dichroscope helps determine the pleochroic property of the gemstone which helps to separate different types of gemstones.
• It also helps to determine whether the gemstone exhibits weak Pleochroism or strong Pleochroism.
• Dichroscope also helps identify some gemstones.
• It also helps to study the optical properties of the gemstone

Polariscope

by Ritika Changrani

Polariscope is an instrument used in gemmology which helps to find if the gemstone is single refractive or double refractive and also allows determining the various crystal axis of the stone.

A Polariscope is used to determine whether the gemstone is natural or synthetic. This is one of the most commonly used instruments by the gemmologists. The use of plane polarized light allows us to see the actual path the beams are taking through the stone.

A polariscope is made of two Polaroid plates that rotate, a power switch, a stone holder and a bottom light source. Two polarized filters or plates are made of polarizing plastic sheets, one is on the top of the instrument, known as analyzer and the other is on the bottom of the instrument, known as polarizer. Polarizer and analyzer have their own vibrational planes. When the vibrational plane of polarizer and the vibrational direction of the analyzer are at right angles to each other, the field between them appears dark. This position is known as crossed position and it is in this position that the gemstones are tested to see if they are isotropic, anisotropic, anomalously double refractive and anisotropic aggregate. The polariscope can be used to determine the optical character as well as the optical sign of the gemstone.

Polariscope can also be used to determine the strains in diamond. It helps in separating natural gemstones from synthetic ones and also helps to distinguish solid inclusions from negative inclusions. Polariscope can also be used for recognizing polysynthetic twinning.

How to use a Polariscope?

• The polariscope is adjusted for cross position by setting the vibrational plane of polarizer and the vibrational direction of analyzer at right angles to each other.
• Turn on the light source and place the gemstone on the rotating platform above the polarizer. The gemstone is rotated in a vertical axis.
• Observe the gemstone through the analyzer and note down the observations
• Based on the observations the nature of the gemstone can be determined as follows.
  • If the stone remains dark throughout the 360 degree rotation, the stone is said to be isotropic or singly refractive.
  • If the stone darkens evenly at every 90 degree interval, the stone is said to be doubly refractive or anisotropic.
  • If the stone appears light throughout the rotation, it is said to be microcrystalline or crypto crystalline aggregate.
  • If the stone shows anomalous double refraction, the stone is singly refractive. Anomalous double refraction may be confusing at times. Hence when there is a suspicion that about anomalous double refraction, it is recommended to orientate the stone in the position where it appears lightest. Then turn the analyzer to 90 degree. If the stone appears lighter than before it is said to exhibit anomalous double refraction and hence is singly refractive. If the stone appears the same, it is doubly refractive.

How does a Polariscope work?

Polariscope works on the principle of plane polarized light. Single refractive gemstones do not break the plane polarized light into various colours. There is only one beam of light coming out. Examples of singly refractive gemstones include diamonds. Double refractive gemstones break the plane polarized light into two paths. One is the ordinary beam and the other is the extraordinary beam. Based on analyzing the transmitted light, the nature of gemstone can be identified. Some of the doubly refractive gemstones are quartz, tourmaline, ruby, zircon, peridot, sapphire and rulite.

Both the filters i.e. polarizer and the analyzer are turned to the dark position. When plane polarized light is passed through the gemstone, the singly refractive stone remains dark since it does not change the path of light. Hence it remains dark when observed in all positions. But in case of double refractive gemstone, when plane polarized light is passed through it, the stone changes the path of the light and hence the direction of the light wave changes. This produces light that is no more polarized. The change in direction of light makes the stone change from light to dark as it is rotated between the Polaroid plates.

Since the polariscope works through plane polarized light, it can be used to test transparent as well as translucent gemstones, but cannot be used to test opaque materials. Once the stone is found to be doubly refractive, a conoscope can help to determine the optic interference figure. Optic interference figure helps to locate the various directions in which the light is traveling through the stone. This information in turn helps to identify the type of gemstone. The different shadow patterns observed through the polariscope helps to determine the crystal structure and diagnostic patterns of the gemstone.

Types of Polariscope

There are two types of polariscope i.e. plane polariscope and circular polariscope. Circular polariscope work on circular polarized light unlike plane polariscope which works on plane polarized light. In a circular polariscope, two quarter wave plates are added to plane polariscope. One quarter wave plate is placed between the polarizer and the gemstone to be tested and the second quarter wave plate is placed between the analyzer and the gemstone. The quarter wave plates produce circularly polarized light. The advantage of using circular polariscope is that it helps distinguish isochromatics and isoclinics. But plane polariscope are more extensively used for testing gemstones.

Refractometer

by Ritika Changrani

A Refractometer is a gemmological tool used to find the ability of gemstone to bend or refract light. This measure is the refractive index of a gemstone.

A Refractometer can be used to measure the refractive index of both singly refractive (isotropic) and doubly refractive (anisotropic) gems. The refractive index helps to determine the composition and physical properties of gemstone. With the help of refractive index, it is easy to identify the type of gemstone. Also, it is possible to estimate the amount of impurity present in the gemstone by comparing the measured refractive index with the standard value of the natural gemstone. With the help of Refractometer, birefringence and optic sign of the gem can also be determined.

How does a Refractometer work?

Refractometer works on the principle of Total Internal Reflection i.e. when light travels from denser material or the material with higher refractive index to rarer material or the material with lower refractive index, the light reflects into the denser material or refracts into the rarer material depending on the angle of incidence of the light traveling. (image courtesy: www.diamondtech.com).

Refractometer is made of a metal case, polarizing filter on the top, glass hemicylinder and viewing lens at the bottom. Natural or white light is passed through the back of the unit. The light travels through the glass hemi cylinder and then to the gemstone which is placed on the glass hemicylinder using a special refractive index liquid, which seals the gemstone to the hemicylinder thereby removing the air in between them. This is done to maintain the same speed of light in both the glass hemicylinder as well as stone so that the refracted rays obtained are accurate. The light is then refracted and sent to the viewing lens through the glass hemicylinder. Through the viewing lens one can see the refracted rays of light. The bending or slowing of light caused by the gemstone appears as the light and dark area combined by thin green line. The green line thus obtained is the refractive index of the gemstone.

A Refractometer is designed such that the incident and exiting rays reach the boundary of hemicylinder at 90 degree. No refraction occurs when the light ray is at 90 degrees to the boundary. Therefore no refracted rays are visible and hence appear as dark areas on the scale. The reflected rays of light appear as the light areas on the scale of the Refractometer. The light and the dark boundary on the scale is the representation of critical angle.

How to use a Refractometer?

• Apply a small amount of refractive index liquid on the metal base near the hemicylinder. Slowly place the gemstone on the refractive index liquid. Then slide the stone to the centre of the hemicylinder. Take care that you don’t apply pressure on the stone as it may damage the hemicylinder. The liquid builds optical contact between the stone and the hemicylinder.
• Close the lid of the Refractometer to avoid external light.
• Place the light source at the back of the Refractometer. Adjust the beam until you see bright light when you observe through the viewing lens.
• Position your eye such that you can see the whole scale through the viewing lens without moving your eye. This is to avoid the parallax error in the reading.
• Move your head up and down until you can see the clear separation of light and dark areas. Note down the reading on the scale where the thin greenish line appears.
• Unscrew the polarizing filter and place it on the viewing lens.
• Take down the reading. Rotate the polarizing filter to 90 degree and note the readings.
• Polarizing filter helps to determine if the stone is singly refractive or doubly refractive. If the stone is singly refractive, only one shadow edge is seen. Note down the readings rotating the stone 135 degrees.
• If the stone is doubly refractive, shadow edges move between two values on the scale. Note down the two values, i.e. the upper and the lower value. Take down four sets of reading by rotating the stone to 45 degrees.
• If one of the reading i.e. either upper reading or the lower reading remains same, the stone is said to be uni-axial. If both the readings change after rotating the stone, the stone is said to be bi-axial.
• The refractive index is the difference between the largest higher reading and the smallest lower reading.
• f the difference is positive, the optic sign of the stone is positive; else the optic sign of the stone is negative.

Tips to Remember while using a Refractometer

• Store the equipment in a dry place to protect the device from moisture which can affect the accuracy of the equipment.
• While you move the gem stone on the glass hemicylinder, take care to move the stone slowly so that no scratch is caused on the hemicylinder. A minute scratch causes can damage it and turns costly to be repaired.
• The refractive index liquid has to be stored and sealed. Avoid its contact with hands or eyes.
• Make sure to use the tiniest drop of refractive index liquid since the liquid itself has its own refractive index and causes total internal reflection between hemicylinder and the liquid. This will in turn affect the accuracy of the reading.
• The contact liquid should be selected with proper care since it sets the limit to the stones tested. The stones with greater refractive index than the contact liquid cannot be tested and hence give negative reading.
• Make sure there is adequate amount of light. White light can be used while testing, but monochromatic yellow light is widely used. White light can give good results for single refractive stones, but for doubly refractive gems sodium light source is the best option. White light, when used with doubly refractive gemstones causes overlap of the refraction readings thereby getting wrong results. Also, the use of sodium light source clearly differentiates the boundary between dark and light areas and hence helps take the readings easily.

Spectroscope

by Ritika Changrani

A Spectroscope helps to identify cut stones, rough stones, mounted stones and unmounted stones to see whether they are real or fakes.

A Spectroscope is a gemological device used to test if the gemstone is natural or synthetic. Spectroscope helps to determine what parts of white light are being absorbed by the gemstone.

White light is made up of seven different colours namely violet, indigo, blue, green, yellow, orange and red, which have different wavelengths and travel with different speed. When white light is passed through a gemstone, a part of it, based on the energy level is absorbed by the gemstone and the remaining colours combine to give a different colour to the stone. When observed through the spectroscope, one can see bands or colours missing. The missing bands are the colours absorbed by the gemstone. The colours absorbed let us know the elements present and the chemical composition in the gemstone which thereby helps in gemstone identification.

Types of Spectroscope

There are two types of spectroscope, diffraction grating spectroscope and prism spectroscope.

Diffraction Grating Spectroscope

Diffraction grating spectroscope implies the principle of diffraction. Diffraction refers to the bending of light waves around a sharp edge or an obstacle by transmission or by reflection. This type of spectroscope is made up of high-dispersion diffraction grating film, movable slits and a photo detector which is used to measure the properties of light within the specified portion of the spectrum. Here light is made to enter a narrow slit. The light is then diffracted by thin high-dispersion diffraction grating film. This causes uniform spectrum image and disperses light into large visible spectrum. The advantage of diffraction grating spectroscope over the prism spectroscopes is that the spectrum is evenly distributed and hence is easier to read. But the spectrum thus produced is not as bright as that produced by prism spectroscope. Diffraction grating spectroscopes do not have an inbuilt calibration scale.

Prism Spectroscope

Prism spectroscope works on the principle of dispersion. Dispersion is defined as the splitting of white light into its constituent colours. Prism spectroscope is made of three optical grade glass prisms placed in optical contact with each other. Most of the prism spectroscopes come with calibration scale. But experienced people make observations without the scale.

In a prism spectroscope, light is made to enter through a narrow slit which is dispersed by passing through a series of prisms. In prism spectroscopes, the spectrum obtained is brighter and faint lines are clearly visible. But the disadvantage with prism spectroscope is that the spectrum is not evenly distributed. Blue parts are more spread and red parts are more condensed. It is difficult to distinguish lines in red part since they are very close to each other. Prism spectroscopes come with focus slide control and light slit control which allows adjusting for the amount of light entering the unit.

How to use a Spectroscope?

• First of all, before testing for the absorption spectra of the gemstones, hold the spectroscope against some different sources of illumination. Holding it against either a fluorescent light bulb or a computer monitor shows clear absorption bands.
• Place the gemstone on a black non-reflective surface may be a piece of black velvety cloth. If the surface is non-reflective, the observations may not be accurate and hence give false readings.
• Place the source of white light such that light enters the pavilion of gemstone at an angle of 45 degrees.
• Place the spectroscope on the other side at the same angle. The angle of 45 degree is chosen since this is the path in which light travels in the longest path picking up most colours.
• The other way of positioning can be by placing the light source and the gemstone in one line such that the light illuminates the gemstone from the back. This helps to view the gemstone in transmitted light.
• Note the observations through the spectroscope. Note down the spectrum seen with red end of the spectrum on the left and violet end of the spectrum to the right. The colours which are absorbed by the gemstone appear as missing bands. The colours absorbed mix up to give the gemstone a different colour.
• Based on the spectrum obtained, you can analyze the composition of the gemstone and hence determine the type of gemstone.
• The spectrum can also be compared with that of a known gemstone to identify if the gemstone is a natural or a synthetic one.

Though spectroscope is one of the important tools in gemology, it should be purchased only when it is extremely necessary. The cost of spectroscope is quite high. Furthermore, only experienced gemmologists can make use of the spectroscope more efficiently. It is difficult for an inexperienced person to study the nature of gemstones using a spectroscope.

Aquamarine Identification

by Erum Qureshi

Aquamarines, belonging to the Beryl family, are relatively easy to identify. Various simple tests can reveal the authenticity of an aquamarine stone.

Summary: Aquamarines, belonging to the Beryl family, are relatively easy to identify. Various simple tests can reveal the authenticity of an aquamarine stone.

Aquamarine – This name is derived from a Latin word, which means water of the sea and so named because of its seawater color.

Most gems have a bluish green color, which was highly prized at one time but has lost its charm lately. There are also clear green Beryls, which are colored by iron instead of the chromium colored emeralds and have a light, clear color, quite unlike the deep green of emerald. The other is the colorless variety, which is named Goshenite (after Goshen County in Hampshire County, Massachusetts). This name, however, is fast becoming redundant, and the name colorless beryl or white beryl is commonly used.

Species

Aquamarine belongs to the species beryl. Beryl is a silicate mineral having chemical formula Be3Al2 (SiO3) 6. Beryl of non-gem quality is mined as the raw material for beryllium, which is now increasingly used in industry. Aquamarine crystals are hexagonal and further, many of them exhibit a tapering form due to erosion.

Hardness

The hardness of aquamarine is 7.5 on the Moh’s scale. Like all Beryls, an aquamarine has a weak basal cleavage and a tendency to brittleness.

Determination of Specific Gravity (SG)

The SG of aquamarine lies between the range of 2.66 and 2.80. Some Madagascan stones have higher densities owing to a trace of alkali metal in their composition. The specific gravity or SG is measured by immersing the stone serially in a set of high-density liquids.

These liquids come in a set of three or five liquids of varying densities. They work on the simple premise that if a stone of lesser density is immersed it will float, that of higher density shall sink and one of equal density will remain suspended.

Refractive Index

The refractive indices for aquamarine vary from 1.572 to 1.590 for ordinary ray and 1.567 to 1.583 for the extraordinary ray.

The refractive index or RI is measured by placing the stone face down with a drop of contact liquid between the polished face of the gem and the glass of the Refractometer. The light rays leaving one medium (air) and entering another obliquely (the stone) will seem to bend a little at the place of contact. This is called refraction of light and the reading is taken herewith.

Pleochroism

Aquamarines exhibit a distinct Dichroism that is the ‘twin colors’, the strength of which depends on the depth of color of the stone – they are deep blue and colorless, the extraordinary ray giving the attractive blue color. The birefringence (difference between the highest and lowest refractive indices) is negative in sign and 0.005 for the lower indices and rises to 0.007 to 0.008 for the stones with higher indices.

Absorption Spectra

When the aquamarine is viewed through a spectroscope, the absorption spectrum it shows (ascribed to iron and not very pronounced) there is a broad band in the violet at 427 nm and a feeble diffuse band in the blue-violet at 456nm. Further, the extraordinary ray, which can be isolated by the use of a Polaroid disc, shows these bands more strongly, and in such conditions a narrow and delicate absorption in the middle green can be observed at 537nm.

Fluorescence

Aquamarine does not exhibit luminescence. All colorless Beryls, like the blue and sea-green stones show a strong greenish-blue color when viewed through the Chelsea filter. In this way, aquamarines can be picked out from a parcel of similar looking stones.

Treatment and look-alikes

Practically all the beautiful blue aquamarines seen in jewelry are greenish-yellow (or even brownish-yellow) stones, which have been heat-treated. The blue color is induced by heating to a temperature between 250 and 720 degree Celsius for a varying period. The resulting color is permanent.

The most effective imitation of the aquamarine is the synthetic spinel colored pale blue by cobalt. These synthetics will have a totally different refractive index (1.782) and SG (3.63) from aquamarine. Gemologists identify these synthetics by placing the suspected stone close to a strong light and viewing it through a Chelsea Filter held close to the eye. The synthetic spinel shows a distinct red under the filter, whereas aquamarine shows a decided green.

Jewellers Loupe

by Ritika Changrani

A jeweller’s loupe is a tool that helps identify the internal flaws, cracks and blemishes on the surface and any other cover ups on the gemstone.

A Loupe is just a magnifying glass. It helps to see the magnified image of gemstone and serves as the identification and grading tool. A Loupe is made of a powerful convex lens. Through a jeweller’s loupe, one can easily see the internal flaws, cracks and blemishes on the surface and any other cover ups on the gemstone. Jeweller’s loupe is the most important test equipment for testing gemstones.

How to Choose a Jeweller’s Loupe?

There are two important factors to be considered while choosing the right jeweller’s loupe. They are the size and magnification of the lens and the optical quality. Loupes are available in different sizes and powers. The standard size used for testing gemstones is of 18mm length with 10X, which magnifies the gemstone to ten times its size. The jeweller’s loupe is available in 20X and 30X, but they are not much used.

The jeweller’s loupe has to be tested for achromatic and spherical aberrations. Chromatic aberration refers to colour fringes of the image due to dispersion caused inside the glass lens. The loupes which are corrected from chromatic aberration are known as achromatic. Spherical aberration refers to the distortion caused due to the differences in refraction on the outer edges of the lens and the rays traveling through the center. The loupes which are corrected for spherical aberration are termed as ‘aplanatic’.

While choosing a jeweller’s loupe it is necessary to check that they are achromatic and aplanatic to get accurate results. In a better quality jeweller’s loupe, three lenses (known as triplet loupe) are used so that the field of vision is in focus to the edges and no false colour is imparted to the eye. The use of three lenses eliminates the pin cushion distortions and chromatic and spherical aberrations.

While selecting a jeweller’s loupe you can easily identify if they are achromatic and aplanatic. Look at the white light through the loupe. If the white light remains white without changing the colour, the lens is corrected for chromatic aberration. Similarly spherical aberration can be tested by looking at the mm squared drawing paper. If they remain squares when looking through the loupe, the loupe is aplanatic.

It is also better to choose a loupe with black framing around the lens, since this avoids reflections which may alter the color of the object under view. Do not opt for golden or other bright coloured frames since they interfere with the colours.

How Does a Jeweller’s Loupe Work?

Jeweller’s Loupe magnifies the image of the stone under test as per its magnification power. Suppose, we are using a 10x loupe, the image is magnified to ten times its original size. This helps us clearly see the flaws, cracks and other distortions in the stone. A triplet loupe consists of one ‘plano-convex lens’, one bi-concave lens and one bi-convex lens. The ‘plano-convex lens‘ is used to eliminate spherical aberration and the other two are used to eliminate chromatic aberration. Depending on the observations, the nature of the gemstone can be detected. Jeweller’s loupe helps to detect diamonds, sapphire, alexandrite, ruby, pearls and many other gems.

How to Use a Jeweller’s Loupe

• Remove your spectacles if you are wearing one. Unfold the jeweller’s loupe.
• Hold the jeweller’s loupe between the thumb and the index finger. Keep the jeweller’s loupe as close to the eye as possible without touching the eyeball. The loupe should be so close that your eyelashes almost brush through it. Once the loupe is close to your eye, see that it is kept stationary by letting the back of the thumb rest against the side of the nose. Keep the remaining three fingers in parallel or just below the index finger. Keep both your eyes open to avoid eye straining.
• Make sure there’s enough light in the place where you are observing the gemstone. Fluorescent and other light bulbs lack some colours which are necessary. Hence sunlight is the best light for the testing gemstone. Diamond lamps can also be used.
• Place a clean white soft cloth on the surface you are using. This helps to avoid scratches on the gemstone and also avoids rolling down in case it slips down. Next, place the stone at a distance of an inch or a little more on the other side of the gem loupe. Peering through the gem loupe, move the stone to and fro until it comes in focus. Keep moving the stone front and back and turn the stone in different directions until the area of the stone is seen with sharp focus.
• Note the external and internal features of gemstones. Look if there is unevenness in colour. If there is unevenness in colour, there is a possibility that the gemstone is dyed or painted to imitate a high quality gem. Many deceptions which cannot appear to the naked eye can be observed through the jeweller’s loupe. If you already have some real gems of similar types, you can observe both and compare them.
• Some of the things you should observe while testing the gemstone through the loupe are :
  • Symmetry: Check if the gemstone has a balanced symmetry
  • Number of facets: Observe whether the gemstone has the right number of facets required for the particular cut
  • Sharpness of the facet edges: Hard stones have sharp edges while synthetic stones have soft edges
  • Inclusions, flaws and bubbles in gemstones: Inclusions, if present, don’t necessarily indicate that the gemstone is bad. Instead, the gemstone can be a natural gem. Flaws in gemstones are not an indication that the gemstone is of low value. Bubbles in gemstone indicate that the gemstone is synthetic.
  • Scratches and cracks in gemstone: Soft stones tend to scratch easily while hard stones crack.