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Crossed polarizers: The value of simple instruments

One can work as well

Sold as a tsavorite garnet, this topaz has a vapour coating that becomes apparent with a white diffuser and proper orientation under the microscope.
Sold as a tsavorite garnet, this topaz has a vapour coating that becomes apparent with a white diffuser and proper orientation under the microscope.

As much as I enjoy polarizing microscopy, I probably use just the single polarizing plate with Mylar (the polarizer) far more often. With the Mylar down, I can use it as a dichroscope by simply rotating the stone (or jewellery) in multiple directions. Using the microscope’s zoom, one can even use this technique for melee gemstones, as long as the polarized light can pass through the stone. If it can’t, switch to the analyzer plate, and while illuminating the melee with fibre optic or a penlight, rotate the stones below the filter or the filter itself if that’s more convenient. As I mentioned previously, flipping the polarizer so the Mylar is on top creates a superb translucent white filter. This position allows me to easily detect colour zoning and is a great background for viewing inclusions. With stones held at oblique angles, the white background provides a good reflective surface for detecting coatings and surface features on gemstones and diamonds. If it’s advantageous, the filter can be positioned above or to one side of the stone to provide the surface reflection needed to detect some surface anomalies.

Polarized light can also be useful in spectroscopy and, of course, with a refractometer. For use with a spectroscope, positioning the polarizer plate with Mylar toward the light source can be very useful for examination of anisotropic gemstones, since the individual rays can be studied separately by rotating the slide. When used with a spectroscope simply as a light diffuser, it can enhance one’s ability to see the absorption spectrum of some gemstones. Rotating an analyzer in the viewing path of a refractometer is a commonly known technique for determining a gemstone’s birefringence.

The fun never stops (unless you let it)

I hope this discussion inspires you to acquire a set of polarizing filters for your microscope and if you’re adventurous, modify one of them with white Mylar. We all look for ways to make our jobs easier and faster; I’ve found this simple gemmological instrument, when properly utilized, can do just that. My hope is readers of this magazine will never stop learning and seeking new ways to solve problems; the savings in time and energy can pay significant dividends and also be a great way to add even more fun to the best job in the jewellery industry.

Mark CartwrightMark T. Cartwright is an accredited senior appraiser, master gemmologist appraiser (American Society of Appraisers), independent certified gemmologist appraiser (American Gem Society), and GIA graduate gemmologist, who has provided gemmological and jewellery appraisal services since 1983. He can be contacted via e-mail at


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