If all stones to be tested in the polariscope were completely transparent, unstrained, flawless, single crystals cut in spheres, the polar scope test  (gem polariscope or gemstone spectroscope) would offer no problems. However, most gemstones are flawed, many are twinned, some are aggregates of tiny crystals, the majority are faceted, and frequently they are mounted. Frosted or poorly polished surfaces may also affect results. A brief analysis of the manner in which light behavior is affected by these conditions is essential to an understanding of the common polariscope reactions.

Singly-Refractive Stones.
Remember that light is not polarized in singly-refractive crystals, it may pass while vibrating in any plane. There are an infinite number of vibration planes at right angles to any given direction of transmission. Within these materials, light is usually refracted or reflected in a normal manner, but not polarized. An exception occurs when stones are under some degree of internal strain.

Doubly-Refractive Stones
The simpler class of doubly-refractive single crystals is that called uniaxial. They are so called because they have one optic axis, or direction of single refraction. The planes of vibration consist of planes oriented as shown in Figure D. Vertical planes converge in the center, so that from the top an infinite number of vibration planes exist about a given point.

CONFIRMATION OF SINGLE OR DOUBLE REFRACTION.
It is clear from the above discussion that the ideal reaction for a singly-refractive stone is to stay dark when rotated in the polariscope's dark position, for the doubly-refractive stone, darkening very 90° is the classic reaction. It is equally clear that the singly refractive stone seldom reacts in the classic manner. In the following paragraphs, methods of checking results are given.

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