AMETHYST (a variety of QUARTZ)
Description: The purple color in amethyst from most localities is unevenly distributed in the individual crystals. This variety of quartz owes its color to gamma irradiation (Berthelot, 1906) and the presence of traces of iron built into its crystal lattice (Holden, 1925). Amethyst can be faceted and is commonly used as a gem material in jewelry.
Chemical Formula: SiO2
Crystal Form: 7 basic crystallographic forms, the hexagonal prism and trigonal rhombohedra are very common
Crystal System: Trigonal
Color: Violet, purple
Streak: Colorless (harder than the streak plate)
Luster: Vitreous (glass-like) in crystals
Fracture: Conchoidal
Cleavage: None - typically breaks with a conchoidal fracture.
Hardness: 7 on Moh’s scale.
Density/Specific Gravity: 2.6 - 2.7 g/cm3
Magnetism: N/A
Taste: N/A
Hydrochloric acid: N/A
Radioactivity: N/A
Fluorescence: N/A
Distinguishing Physical Properties:
Hardness: (easily scratches glass, also harder than steel)
Luster: glass-like
Fracture: conchoidal fracture in crystals, in massive specimens the fracture often looks irregular to the naked eye, but still conchoidal at high magnification.
Cleavage: poor to indistinct cleavage
Photograph Attribution: Amethyst specimen photograph by Sean C. Murphy, 2020.