Pleochroism in the Determination of Gem Minerals

Abstract

As is well known, pleochroism, or the varied colors exhibited by some minerals when examined in transmitted plane polarized light, is a property of marked value in distinguishing various gem stones. With pleochroism, the effect due to absorption, or relative intensity of the light passing through the mineral in various directions is also to be carefully observed. The use of the instrument called a dichroscope in the detection of pleochroistn in gems is rather common, but does not serve the purpose as well as the Nicol prism or polarizer forming part of the substage of a polarizing microscope. With this last apparatus, if correctly designed, it is possible to establish the relations which exist between the pleochroism and absorption on the one hand, and the directions of vibration of the light passing through a birefringent mineral on the other, besides many other properties such as inclusions, fractures, flaws, cleavage planes and several optical characteristics, which serve to establish the diagnosis and also the qualities of the gem under observation. The relations between the directions of vibration and the nature and intensity of the colors constituting the pleochroism usually exhibit marked discontinuities between many of the gem stones and are thus very valuable in determinative work, particularly in the case of cut stones where the obtaining of fragments is usually impossible without defacing the gem.