Amethyst
Amethyst is a violet variety of quartz. The name comes from the Koine Greek αμέθυστος amethystos from α- a-, "not" and μεθύσκω (Ancient Greek) methysko / μεθώ metho (Modern Greek), "intoxicate", a reference to the belief that the stone protected its owner from drunkenness. Ancient Greeks wore amethyst and carved drinking vessels from it in the belief that it would prevent intoxication.
| Amethyst | |
|---|---|
| General | |
| Category | Silicate mineral |
| Formula (repeating unit) | Silica (silicon dioxide, SiO2) |
| Crystal system | Trigonal |
| Crystal class | Trapezohedral (32) |
| Space group | P3221 (no. 154) |
| Identification | |
| Color | Purple, violet, dark purple |
| Crystal habit | 6-sided prism ending in 6-sided pyramid (typical) |
| Twinning | Dauphine law, Brazil law, and Japan law |
| Cleavage | None |
| Fracture | Conchoidal |
| Mohs scale hardness | 7–lower in impure varieties |
| Luster | Vitreous/glassy |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.65 constant; variable in impure varieties |
| Optical properties | Uniaxial (+) |
| Refractive index | nω = 1.543–1.553 nε = 1.552–1.554 |
| Birefringence | +0.009 (B-G interval) |
| Pleochroism | Weak to moderate purple/reddish purple |
| Melting point | 1650±75 °C |
| Solubility | Insoluble in common solvents |
| Other characteristics | Piezoelectric |
Amethyst, a semiprecious stone, is often used in jewelry.
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