Sugilite

Chemistry: Sodium potassium ferric silicate
Crystal system: Hexagonal; massive.
Color: Opaque; light to dark violet to reddish purple (Mg); color-change (blue purple: daylight; reddish purple: incandescent light).
Hardness: 6 – 6.5
Cleavage: Fracture: granular
Specific gravity: 2.74 mean; varies.
Refractive index: 1.607 – 1.610 (1.61 mean); Uniaxial negative.
Luster: Vitreous to waxy.
Dispersion:-
Dichroism: -
Occurrence: Vein infillings intergrown with other minerals; South Africa, Nambia.

Notes
Discovered in 1976; trade names include Royal Lavulite, Royal Azel; ornamental bead, cabochon, carvings; mineralogical name: manganaoan sugilite; constants may vary due to the presence of other manganese minerals; spectrum: broad absorption at 620 and 480nm, 4 sharp bands in blue and violet.