Amethyst
Amethyst cluster from Magaliesburg, South Africa.
General
Category Quartz
Formula
(repeating unit) Silica (silicon dioxide, SiO2)
Identification
Color Purple, violet
Crystal habit
6-sided prism ending in 6-sided pyramid (typical)
Crystal system
rhombohedral class 32
Twinning
Dauphine law, Brazil law, and Japan law
Cleavage
None
Fracture
Conchoidal
Mohs scalehardness
7–lower in impure varieties
Luster
Vitreous/glossy
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
None
Melting point
1650±75 °C
Solubility
insoluble in common solvents
Other characteristics Piezoelectric