About the Optical Properties of Refraction and Polarization of Light in Precious Gem Stones

When a ray of light passes from one medium to another, or from the air to a crystal it is bent or refracted; this is called single refraction and takes place in the diamond, spinel, and garnet.

Most of the other transparent precious stones possess double refraction--that is, the ray of light enters the crystal and divides into two parts, one following the ordinary laws of refraction, while the other part or extraordinary ray does not obey the usual law.

There are precise methods for measuring the indices of refraction, but they are not applicable to polished gem stones.

Polarization is a peculiar modification which, under certain conditions, a ray of light undergoes. This property is easier to observe than double refraction.

If from a transparent prism of tourmaline two thin plates are cut, parallel to its axis, they will transmit light when they are placed above each other with the chief axis of each in the same direction.

When one of the plates is turned at right angles to the other, no light, or but very little, is transmitted, so that the plates appear black.

In passing through the first slip, the rays of light have acquired a peculiar property, which renders them incapable of being transmitted through the second, except when the two are held in a parallel position, and the rays are then said to be polarized.