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Polariscopes are
a very useful, simple and inexpensive to make piece of gemological equipment.
They are used to tell glass from gem materials synthetic spinel from all
other materials, singly refractive from doubly refractive, crystaline from
cryptocrystaline material, doublets and triplets from other gems, identify
yellow Verneuil corundum (Plato test-see Liddicoat -GIA), determine quartz
definitively from other materials, tell whether a transparent gemstone is
biaxial or uniaxial in its crystal system. This is pretty good or equipment
that may be as simple as a camera lens and a pair of polaroid® sunglasses.
This is one piece of gemological equipment that is fairly expensive
and can be made very easily and cheaply. Prices for polariscopes start
around $65.00 and go up. Hanneman has a very basic model (two sheets of
polaroid and a tube) for around $4.00 and supplies two 2" x 2" sheets
of polaroid filter material for $1.00. I have made one using two polaroid
sunglass lenses on a stick and a student I had (Dennis O'Hanley) used
sunglasse lenses and an empty underarm deodorant tube container with holes
cut in top and bottom to fit the lenses. He cut an access port in the
side to permit manipulation of the gemstone (1.3). One could rotate the
top of the deodorant tube to cross the filters as needed. My own polariscope
was made using two 49mm camera polarizing filters ($14.00 total) and a
length of appropriately sized PVC pipe. Two rings were cut off the end
of the pipe, small segments cut out of these rings so that they could
be forced together and slid into the pipe where they formed ledges upon
which the camera filter sit. An access port was then cut in the side of
the PVC pipe. An advantage of using commercial lenses is that they rotate
freely when in place. The GIA microscope immersion cell (about $10.0)
just fits in the filter (49 mm) so that several gems at once may be examined
in polarized light conditions by rotating the immersion cell. It is made
in such a way that the glass of the immersion cell does not interfere
with polariscope use.
A light source would be a flashlight, which fits in the tube also. I
use my microprojector as a light source. A paper disc is placed just below
the lower polarizing filter to diffuse the light somewhat. A strainless
glass sphere on a rod is available for resolving interference figures
of gems with the polariscope. This would identify quartz and tell whether
a gem is biaxial or uniaxial in crystal system. GIA and Hanneman sell
them.
Mr. Hamilton Stitt, F.G.A. described a very simple field polariscope
in the Journal of Gemmology. A polaroid filter is placed over the glass
in the well of a small flashlight. One then dons polaroid sunglasses and
has an instant polariscope with an adjustable distance between lower (flashlight
well) and upper (sunglasses) filters to examine stones. The rim of the
flashlight prevents stone loss and interference figures may be resolved
with a 10X hand lens. This is really a pocket polariscope.
Using the polariscope To use the polariscope the top polarizing filter is turned to the extinction
position (dark) allowing the least amount of light to pass through the
two filters. The stone is placed between the two filters and rotated around
a vertical axis. If it darkens evenly at exactly 90o intervals (4x per
rotation) it indicates double refraction. It is important to test each
stone in more than two positions as a doubly refractive stone may have
one or two directions of single refraction (optic axes) in it. If it remains
dark throughout its rotation single refraction is indicated. Cryptocrystalline
material remains bright during rotation (jade, chalcedony) but some glass
with textured backs does the same so it is best to use this test for transparent
stones only. Note that large numbers of doubly refractive inclusions would
have a similar effect. Brilliant cut stones should not be tested table
up or down as they may reflect out all the light entering the table giving
a false reading.
Anomalous double refraction may occur in singly refractive materials
with internal strains such as diamond, garnet, synthetic spinel, amber,
plastic, opal and glass. The extinction patterns here however do not occur
at precisely 90o intervals although rarely they may be close. Plastic
and amber may show bright interference colours. Glass may show a characteristic
cross like shadow, or two approaching bars which almost form a cross during
rotation. Synthetic spinel shows a characteristic 'tabby' extinction,
a sort of fine, mottled cross hatching of parallel silk-like lines which
change during rotation. In a doubly refractive stone interference colours
will appear when one is within a few degrees of an optic axis. To produce
diagnostic interference figures one obtains the optic axis position and
placed a condensing lens above the stone or in contact with it. This can
be a 10X lens (viewed from 18" or so distance), a drop of viscous liquid
or a strainless glass sphere. This will divulge whether the stone is uniaxial
or biaxial.
The characteristic interference figure for uniaxial gems is:
Quartz has a special variety of this uniaxial figure called a bullseye
uniaxial figure and this is, if seen clearly, diagnostic for quartz. The
centre circle is often red.
Biaxial figures vary somewhat but are quite distinctive having usually
only two arms or brushes from a centre or oval:
It may be noted that cabachon gems or beads often need no condensing
lens as they function as one themselves. Interference figures may be resolved
more easily if the gem is immersed in water or bromoform (toxic). Be sure
that the transparent container for your liquid does not itself add shadow
lines to your image, especially if merely testing for double refraction.
Interference figures may be used to distinguish between: moonstone (orthoclase)
and chalcedonic quartz which shows no figure resolution, topaz and tourmaline,
andalusite and tourmaline, corundum and chrysoberyl.
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