Tuesday, July 29, 2008

Pleochroism

As explained before, different colors can often be observed in double refracting gems by looking at them from differ ent directions. Any color differences seen must be remembered. This dichroism or pleochroism can be seen, and the colors compared directly, without the necessity of relying on memory, by using a dichroscope. In its simplest form, this instrument uses two small squares of Polaroid sheet which are fastened with their edges touching but with their polarizing directions set at right angles to each other. The gem is viewed through the Polaroid against a strong light. If dichroism is present, the color of the gem portion seen through one section of the Polaroid will differ from that portion seen through the other. By turning the gem in several directions, a third color may possibly appear. If there are observable color differences, the gemstone is doubly refracting and cannot be an isometric mineral or glass or plastic. If only two colors are visible, very likely the mineral is tetragonal or hexagonal. Three colors will almost guarantee that the mineral is orthorhombic, monoclinic, or tri-clinic and three stone wedding rings.
Sometimes dichroism and pleochroism are a little difficult to see because the color differences may be subtle enough to escape visual detection. By itself, the use of the dichroscope is not a positive means of identifying gemstones, but it does add helpful information to that obtained by other tests.

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