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.