In crystal optics, a number indicating the speed of the velocity of light in a vacuum or air to its velocity in a given crystal. The refractive index (RI) of a crystal can be expressed as ratio of the sine of the angle of incidence to the sine of the angle of refraction from Snell's law for any two media. Its conventional symbol is n. The higher refractive index of a stone, the greater brilliance, such as diamond with RI: 2.4173 for sodium light at 589.3 nm. When a velocity of light is decrease in a gemstone (dense material) it is always inversely proportional to the optical density. In single refractive or uniaxial mineral the indices are "w" for ordinary-ray and "e" for extraordinary-ray. In the double refractive biaxial stones the indices are "a" a minimum value, "b" an intermediate value and "x" a maximum value.
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