Ettringite is an extremely rare gem and a member of the Ettingite Group of minerals that includes Charlesite, Ettringite, Sturmanite and Thaumasite. Ettringite crystals are usually very small so gems are very small. Ettringite is closely related to and difficult to distinguish from Sturmanite. Ettingite crystals are usually found as colorless but have been found as yellow to pale yellow from the source at the N’Chwaning mine, Kuruman district, Cape Province, South Africa. This location is currently the only source for gem quality crystals. Other locations of Ettringite only produce minute crystals too small for faceting.

Category: Sulfate minerals
Formula: Ca6Al2(SO4)3(OH)12·26H2O
calcium tri-sulfoaluminate hydrate
Crystallography: Hexagonal – Dihexagonal Dipyramidal
Crystal Habit: Crystals prismatic and striated, to 20 cm, commonly unterminated, rarely dipyramidal; may be acicular, fibrous or cottonlike.
Twinning: n/a


Cleavage: [1010] Perfect
Fracture: Uneven
Tenacity: Brittle
Hardness (Mohs): 2.0 – 2.5
Density: 1.77 (g/cm3)
Luminescence: None
Radioactivity: Not Radioacitve
Other: Partially soluble in water; partially dehydrates on exposure, becoming opaque


Color: Yellow, pale Yellow, Colorless, milky White; turns White upon partial dehydration
Transparency: Transparent to Opaque
Luster: Vitreous
Refractive Index: 1.470 – 1.491  Uniaxial ( – ); changes to Uniaxial ( + ) upon dehydration
Birefringence: 0.0210
Dispersion: n/a
Pleochroism: n/a