???Quartz
Quartz is a silicate and one of the most abundant minerals found in the Earth’s crust. Its chemical formula is SiO2 and its transparent crystals have a moderate refractive index. Its raw crystal growth typically consists of a 6-sided prism ending in a 6-sided pyramid. Being a crystal, quartz has modest birefringent properties. Its ordinary ray (index = 1.54425) and its extraordinary ray (refractive index = 1.55338) of electromagnetic radiation experience a refractive index difference of only 0.009 at 590 nanometers.
The word quartz usually refers to the natural crystal or mineral as opposed to the phrase “fused quartz” which refers to the glass that is created out of a manufacturing process which involves heating quartz crystals to temperatures of around 2000 degrees Celsius (which has lower indices). Pure quartz is colorless and transparent, though impure forms have recognizable names such as citrine (pale yellow or brown with a refractive index of ~1.55) and amethyst (purple with an index of ~1.54). Macrocrystalline (large crystal) varieties can be transparent to translucent. Cryptocrystalline (crystals too small to be seen by a microscope) varieties can be translucent to opaque.
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Refractive Index Reference - Gorachand Ghosh, Opt. Commun. 163, 95-102 (1999)
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