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Ruby Identification @ Jewel Info 4 U
By: Erum Qureshi
Identification of Rubies
Summary:
There are numerous tests conducted
to identify true rubies. Various
characteristics and traits are
judged by a gemologist to
authenticate and grade a ruby.
Introduction
The first criteria a gemologist
looks for in a stone purported to be
a ruby is the color - a strong red
color, and a stone belonging to the
corundum species will be the first
very basic factors, which will
decide if the given stone is indeed
a ruby and whether it ought to be
subject to further examination and
tests to prove beyond doubt that it
is indeed a ruby. Tests for
hardness, for measuring the specific
gravity of the stone and the
refraction of light are just some of
them.
Physical Properties of Rubies
Species
 Corundum as a mineral would be
colorless if pure. However the
presence of one or more metallic
oxides present in the stone as
impurities gives corundum its
colors. All stones belonging to this
species ranging in varying shades of
red to a deep crimson are called
rubies.
Light red, pink or even violet
corundum are usually called
sapphires because if they were
grouped as rubies they would be
stones of an inferior quality.
Hence, besides red, all other colors
of corundum are properly termed as
sapphires with color prefix.
Corundum is found in many colors
besides red (ruby) and blue
(sapphire). Colorless, yellow,
violet, green and even brown colors
may be found. The name ruby
literally means red (derived from
the Latin word - 'ruber'.) Rubies
vary in shades from varieties of red
to a deep crimson due to traces of
chromic oxide (Cr2O3) in its crystal
structure. The amount of chromic
oxide (usually 1-3%) determines the
depth of color in the stones and
traces of iron in the ferric state
gives them a brownish tinge. The
ideal color for a ruby would be a
strong traffic signal red.
HardnessAlthough corundum is a hard mineral
(rating 9 on the
Mohs scale of
hardness) care needs to be taken
during testing as they are slightly
brittle and if dropped on a hard
surface or given a sharp blow, are
prone to develop internal flaws or
fractures.Step by step tests to determine Ruby's
authenticity
Determination of Specific Gravity
(SG)Each gemstone has its own specific
gravity which is ratio of its weight
to the same volume of water.
 The specific gravity of purest
corundum is around 3.98, this value,
way beyond the range of commercially
sold heavy liquids usually used to
determine Specific gravity.
Therefore a special solution called
"Clerici" is used. This solution
contains very soluble thallium
salts: the 'formate' and the 'malonate'.
The Clerici solution at a specific
gravity of 4.00 is dense enough to
float corundum at room temperature.
Thus, in spite of being hazardous
and viscous as compared to other
high density liquids, it is used to
measure the specific gravity of Ruby
and Sapphire.
The Clerici solution is an aqueous
solution and can be diluted with
distilled water. Ruby when suspended
in a beaker of Clerici solution has
to be clean and dry and so do the
tweezers and other apparatus used
for the test. Care needs to taken
that these are free from other
organic solutions before immersing
the gem in the solution.
Natural ruby will have an SG
approximating 3.99 although there is
a variation for stones mined from
different localities. Once the SG is
confirmed to be that of corundum,
the gemologist moves on to the next
step.
Determination of
Refractive IndexLight rays leaving one medium and
entering another obliquely seem to
bend a little at the place of
contact. This is called refraction
of light.
During gem testing, measuring the
refractive index or RI helps the
gemologist observe the indices of
refraction of light on a particular
species of gemstone. Spot readings
are taken for cabochon cut rubies;
faceted stones are placed facedown
on the glass of a
Refractometer with
a small amount of contact liquid and
the reading is taken. The RI of ruby
will be between 1.76 and 1.77.
The refractive index can sometimes
be a decisive factor for
successfully eliminating a ruby
look-alike like tourmaline which
will have a much lower RI of 1.61 to
1.66.
Double Refraction TestWhen a ray of light enters a crystal
having a Trigonal system in
directions other than the optic
axis, it splits into two distinct
rays. This is known as double
refraction. The Ruby, being of the
Trigonal crystal structure shows two
indices of refraction in a
polariscope. These are known as the
ordinary ray and the extraordinary
ray.
This can be useful in identifying
gemstones, the difference between
the highest and lowest refractive
index is known as birefringence.
Ruby has a birefringence of - 0.008,
with a negative sign because the
ordinary ray is of greater value.
Pleochroism TestBecause the ruby is doubly
refractive (it has two rays), each
of these rays absorbs light
differently and this has an effect
on the stone's color. When it is
turned, it shows a slightly
different shade of color and this
effect is known as
Dichroism.
The ordinary ray in ruby showing a
deep purplish red is the most
attractive and desirable for this
species.
Determination of
Absorption SpectraThe absorption of colored stones is
in well marked bands or finer dark
lines which cross the otherwise
continuous spectrum, obliterating
certain colors or wavelengths when
white light is transmitted through
or reflected from the stone.
The spectroscope is an instrument
that determines the wavelength of
the absorbed light. The wavelength
is measured in nanometers (symbol
nm) or angstrom symbol ?. The
spectrum most common to Ruby is the
chromium spectrum. It is
characterized by general absorption
in the deep violet, transmission in
the blue, an absorption band in the
green and transmission in the orange
and red.
Fluorescence TestThe luminescence of rubies whose
coloring agent is chromium is a
strong crimson / orange light when
subject to any kind of radiation
(short-wave, long-wave or even blue
light). When viewed through the
Chelsea color filter, this
fluorescent red is visible. It is
this property that can be used to
distinguish between garnets and
rubies - garnets do not fluoresce.
Microscopic ExaminationWithout doubt the final and most
determinative test to decide the
authenticity of any gemstone is its
examination under a microscope.
Gemologists rely on their expertise
and experience to spot the
inclusions before taking the final
call on the authenticity and grade
of a gemstone. Inclusions
(irregularities in the crystal
structure and internal flaws) in
ruby are straight angular growth
lines following the crystal faces.
Included are rutile needles or
hematite plates called 'silk', which
bring about a soft sheen in the
stone.
 When such a stone is cut en cabochon
these rutile needles or 'silk'
brings about the rare cat's eye
effect or the very desirable
asterism (a six rayed star that
shimmers above the stone's surface
when it is moved). Rhombohedral
twinning and boehmite needles are
inclusions exclusive to natural
corundum and have never been
observed in flux grown synthetic
corundums. Inclusions typical to
synthetic ruby are curved growth
lines (thin striae or bands), gas
bubbles and dense clouds of
extremely fine rutile silk in star
stones. These clouds do not show the
straight angular zoning patterns
that natural stones show, they have
curving bands instead.
Besides the observing the color,
there are an array of tests to
identify a given stone as a ruby.
These tests, conducted by trained
gemologists under strict guidelines
using highly sophisticated equipment
will establish beyond doubt the
identity of any given gemstone. As
it is next to impossible for an
untrained eye to distinguish between
synthetic and natural stones, or
even ruby or a look alike from a gem
of another species, these methods
are valuable tools that help in
identifying gemstones and making a
correct, informed decision about
their purchase or use.
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