Each gem mineral interacts with light in constant and measurable ways. These properties can be used in its identifications. Light entering a mineral is slowed down and refracted from its original path in air. Cubic and non-crystalline minerals are singly refractive, meaning that light is slowed and refracted equally in all directions through the minerals. Minerals crystallizing in all other systems are doubly refractive. In doubly refractive minerals the light entering is split into two rays that are differently slowed and refracted and which travel along different paths through the crystal structure. Double images may be seen through minerals such as calcite where these differences are large.
A constant mathematical relationship, called the refractive index, exists between the angle at which light strikes a mineral and the angle of refraction. Singly refractive minerals have only one refractive index. Doubly refractive minerals show a range of refractive indices between an upper and lower limit characteristic of each mineral. The numerical difference between the maximum and minimum indices is called the double refraction or birefringence. Birefringence can only occur in doubly refractive minerals and is given the abbreviation DR.
The refractive index of most gemstones can be measured accurately on a standard gemmological refractometer if the gem has at least one flat polished surface. This surface is placed on a glass prism in the top of the refractometer and light, usually of a single yellow wavelength, is shone into the instrument. If the gem have as lower RI than the prism glass, some of the light is refracted out through the gem and the remainder is reflected back into the refractometer. When you look into the eyepiece, the junction between reflected and refracted light appears as one or two shadow edges depending on where the gem is singly or doubly refractive. The position of the shadow edges depends on the refractivity of the gem, so they can be read off against a scare to give the RI of the gem within the range of 1.4 to 1.8. Many gems have a unique combination of refractive index and birefringence.
A new instrument called the Brewster Angle Meter was introduced in 1999. With a laser as the light source the instrument is not restricted to a range of 1.4 to 1.8 and can be used to find RIs of gems outside this range. This is particularly useful for diamond and diamond imitations such as cubic zirconia and zircon that have RIs higher than 1.8.
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