Amethyst

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Amethyst
Amethyst. Magaliesburg, South Africa.jpg
Amethyst cluster from Magaliesburg, South Africa.
General
CategorySilicate mineral
Formula
(repeating unit)
Silica (silicon dioxide, SiO2)
Crystal systemTrigonal
Crystal classTrapezohedral (32)
Identification
ColorPurple, violet
Crystal habit6-sided prism ending in 6-sided pyramid (typical)
TwinningDauphine law, Brazil law, and Japan law
CleavageNone
FractureConchoidal
Mohs scale hardness7–lower in impure varieties
LusterVitreous/glassy
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity2.65 constant; variable in impure varieties
Optical propertiesUniaxial (+)
Refractive indexnω = 1.543–1.553
nε = 1.552–1.554
Birefringence+0.009 (B-G interval)
PleochroismNone
Melting point1650±75 °C
Solubilityinsoluble in common solvents
Other characteristicsPiezoelectric

Amethyst is a variety of quartz. It is the birthstone of February and is believed to prevent intoxication.