Diamond is a singly refractive material. The diagnostic method based on this property allows one to unambiguously distinguish a diamond from any doubly refractive imitation but fails to discriminate between a diamond and a singly refractive imitation. The optic character of a stone is its indirect diagnostic feature. To test the optic character of the studied sample, polariscopes are used.
The double refraction of the studied sample is often best observable when laterally viewing the stone (from the side of its girdle). In this case, a composite stone may be singly refractive within one portion and doubly refractive within another.
The crystals of cubic syngony (including diamond) and glasses are singly refractive. However, they often show anomalous birefringence when viewed between crossed polarization filters. This birefringence is due to stresses in the crystal structure of the material. In such a case, instead of clear successive brightening and darkening of the studied sample, alternating dark and bright spots or stripes, or dark grating are observed. Synthetic spinel always shows anomalous effects, such as wavy darkening. Due to stresses that often arise in glasses, a dark cross is often observed in a glass.
Being a doubly refractive mineral, moissanite brightens and darkens four times when rotated between two crossed polarization filters. This feature is enough to distinguish between moissanite and diamond.
If the polariscope test confirms the double refraction of the studied sample, this is enough to conclude that the sample is an imitation. Other results of the test do not allow one to make a final conclusion.