Amethyst

A variety of Quartz

A violet to purple variety of quartz that owes its color to gamma irradiation (Berthelot, 1906) and the presence of traces of iron built into its crystal lattice (Holden, 1925). The irradiation causes the iron Fe(+3) atoms that replace Si in the lattice to lose an electron and form a [FeO4]0 color center (Lehmann and Moore, 1966).
The color in amethyst from most localities is unevenly distributed in the individual crystals. In amethyst geodes it is often most intense in the growth zones under the rhombohedral faces (at the tips). Occasionally the color is deeper under either the r or z rhombohedral faces, giving the crystal a pinwheel appearence when viewed from the top. In prismatic crystals the color may appear in phantom-like thin layers, while in scepters and skeleton quartz the color is often concentrated along the edges, and accompanied by smoky zones. Despite the intense color, the content of iron occupying Si positions in amethyst is rather low, in the 10-100 ppm range (Dennen and Puckett, 1972).

Name

From Greek "a-methystos", meaning not drunk.

Physical Properties

Lustre: Vitreous
Colour: Violet – purple. Pleachroic: Blue – violet to purple

Chemical Properties

Formula: SiO2
Elements listed: O, Si

Synonyms

Amethystine agate, Amethystine quartz, Bishops Stone and Lavendine

Varieties

Burnt amethyst, and Hair Amethyst