Gems that appear one colour from one direction, but exhibit one or more other shades or colours when viewed from different directions, .ire known as pleochroic. Amor­phous or cubic stones show one colour only; tetragonal, hexagonal, in trigonal stones show two colours (dichroic); orthorhombic, mono-rlinic, or triclinic stones may show i luce colours (trichroic).

When a ray of light meets the surface of a polished gemstone, some light is reflected, but most passes in. Because the gem has a different optical density from air, the light slows down and is bent from its original path (refracted). The amount of refraction within a gem is called its refractive index (RI) ,and with the DR (below), can be used to help identify the stone.

When viewed through a refractometer (far right), cubic minerals like spinel are singly refractive, showing a single shadow edge; doubly refractive minerals like tourmaline split light rays in two, producing two shadow edges. The difference 1'i-fween the two gives the "birefringence" (DR).

FLOUROSCENCE

Many materials are fluorescent. That is, when exposed to ultraviolet light or X-rays, they transform some of the incoming energy into visible light. The color and intensity of the fluorescence is often indicative, but not conclusive, of the identity of the material. For example, natural yellow sapphires from Ceylon show a distinctive apricot-colored fluorescence, while synthetic yellow sapphires generally show no fluorescence or a dull red when exposed to long-wave ultraviolet (UV) light. Most natural emeralds are inert (non-fluorescent) under long-wave UV, and most synthetic emeralds show a moderate to strong red fluorescence. Because of the prominent exceptions, this test alone is inconclusive.

Flouroscence in a synthetic Emerald