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Lab identification of Sapphire @ Jewel Info 4 U
By: Erum Qureshi
Summary: Sapphire, derived
from the Greek word 'sapphirus',
literally means blue. Many people
will find it difficult to believe
that ruby, with its intense red
color and the royal blue sapphire
are the same mineral corundum
composed of oxygen and the light
metal Aluminium.
Ideal sapphires are stones of an
intense blue verging on the violet.
Large sapphires are more common than
large rubies. Let us look at the
physical properties of the mineral
species corundum to which the
sapphire belongs.
Species
 Metallic oxides are responsible for
imparting color to this mineral. If
not for these impurities present in
the crystal structure of corundum,
it would be colorless. Red stones of
the corundum species are termed as
rubies and all other colors are
known as sapphires with the color
prefix.
Corundum is found in many colors
besides red and blue. Colorless
Sapphire is called leuko-sapphire,
yellow, violet, green and even brown
colors may be found. Also sometimes
found is a very rare and highly
valued orange color, known as 'padparadscha'
(from the Sinhalese word for 'lotus
flower'). Occasionally
color-changing sapphires can be
found; asterism and star-stones come
in almost all colors.
Hardness
Hardness is the same as ruby (9 on
the Moh's scale) and also differs
differently in different directions
(an important factor which need to
be kept in mind during cutting).
Determination of Specific Gravity
Specific gravity of a gemstone is
the ratio of its weight to the same
volume of water. Since there is no
definite demarcation between ruby
and sapphire, the same tests for
measuring specific gravity are used
as those for ruby. The Clerici
solution is used to measure the
specific gravity of sapphire as this
solution is the heaviest among the
high-density liquids available. The
stone when suspended in the Clerici
solution should be clean and dry as
the tweezers and other apparatus
used for the test. Care is should
also be taken that these are free
from other organic solutions before
immersing the gem in the solution.
Natural sapphire will have an SG
approximating 3.99. An exception is
iron-rich yellow, green and
blue-green sapphires, which may have
specific gravity as high as 4.00.
Once the SG is confirmed to be that
of corundum, the gemologist moves on
to the next step.
Refractive Index
Light 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. The
refractive index for sapphire is
measured exactly in the same way as
that of ruby, both being of the same
species, their RI is the same. Spot
readings are taken for cabochon cut
sapphires; 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 sapphire will be
1.76 to 1.77.
Double Refraction
When 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 Sapphire, 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. Birefringence is the difference
between the highest and lowest
refractive indices. Sapphire has a
birefringence of - 0.008, with a
negative sign because the ordinary
ray is f greater value.
The fact that there are two rays in
these stones has an effect on color,
for each of these rays absorbs light
differently and as the stone is
turned it shows a slightly different
shade of color. This effect known as
Dichroism, is seen fairly well in
sapphires of all colors except in
colorless and yellow stones. The
most attractive color is that of the
ordinary ray in sapphire, which is a
deep royal blue.
Absorption Spectra
The 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 blue, green and
yellow Sapphire is the iron
spectrum. It consists of three lines
of increasing intensity at 451, 460
and 470 nm. The absorption spectra
of sapphire (in nanometers):
Sapphire ,blue (from Australia) :
471, 460, 450.
Sapphire, blue (from Sri Lanka) :
(450)
Note: Strong absorption lines are in
bold letters; weak ones are in
parenthesis.
Fluorescence
In the case of blue sapphires, UV
fluorescence is practically
non-existent. Except maybe Sri
Lankan sapphires, which have
chromium traces. These show a red or
orange glow under long wave and weak
blue glow under short wave.
Microscopic Examination
Without 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. The best stones are
those with no major color zoning
problems, which are well-cut, eye
clean and most importantly of an
intense blue color. Inclusions
(irregularities in the crystal
structure and internal flaws) in
sapphire are straight angular growth
lines following the crystal faces.
Included rutile needles or hematite
plates are 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 stones 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 sapphire 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 but have
curving bands instead.
Treatment and look-alikes
Through heat treatment at
temperatures of about 1700-1800
degrees C, some cloudy sapphires,
non-distinct in color, can change to
a bright blue permanent color.
Sapphires can be confused with
benitoite, indicolite, iolite,
spinel, tanzanite, topaz, zircon and
even blue glass. Some imitations are
made using doublets, blue cobalt
glass with a crown of garnet or a
crown of green sapphire and a
pavilion of synthetic blue sapphire.
Today, synthetic sapphires having
properties identical to the natural
stone are being produced
commercially.
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