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See Alexandrite Tsarstone collectors guide, Definition of Index of refraction, 
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 (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\rquote 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 ) (as of ).
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Index of refraction
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Index of refraction
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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\rquote 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 \ldblquote w\rdblquote  for ordinary-ray and \ldblquote e\rdblquote  for extraordinary-ray. In the double refractive biaxial stones the indices are \ldblquote a\rdblquote  a minimum value, \ldblquote b\rdblquote  an intermediate value and \ldblquote x\rdblquote  a maximum value.
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