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.