In some doubly refracting crystals the two oppositely polarized beams are of different colors, so upon double refraction and polarization depends the property of many gems which is called pleiochroism.
Pleiochroism.
The dichroiscope is a handy little optical instrument, that will readily serve to distinguish the diamond, spinel, or garnet (all singly refracting minerals) from the ruby, beryl, or any of the doubly refracting stones. This instrument consists of a cleavage rhombohedron of Iceland spar, fastened in a brass tube about 2½ inches long, and ¾ of an inch in diameter. A sliding cap at one end has a perforation ⅛ of an inch square, and at the other end is a lens which will show a distinct image of the square opening when the cap is pulled out about ¼ of an inch.
Fig. 1.
The pleiochroism of many stones can be determined at a glance with the dichroiscope.
When a stone is examined by means of the dichroiscope, it will show two images of the same hue, or of different hues, these square images ([fig. 1, A]) forming a right angle and being more distinct when viewed from one part of the stone than from another.
When the images are identical in color, the specimen may be a diamond, garnet, spinel, or glass. Should a red or ruby spinel approach the ruby in color, a quick and satisfactory test can be made with the dichroiscope, as the spinel will show two images of one color, while the ruby will show one image of aurora red and one of carmine red.
The dichroiscope is inexpensive, costing but a few dollars, and is very useful for rapidly deciding the species of many stones. The following is a partial list of doubly refracting stones and their twin colors.