Diopside - CaMgSi₂O₆

Habit: Light to dark green, blue, brown, colorless, snow white or grey. Prismatic crystals with nearly square cross-sections; also granular, columnar or lamellar massive. Vitreous to dull luster; transparent to opaque. Whitish green streak.

Environment: Typical of metamorphosed siliceous calcium- and magnesium-rich rocks of pyroxene-hornfels or epidote­amphibolite facies. Also found in some kimberlites, skarns and calcium- and magnesium-rich gneisses and schists; less common in alkalic olivine basalts and andesites.

Etymology: From the Greek words for "double" and "appearance," apparently because there are two possible orientations of the prism zone.

Diopsides enriched with chrome tend to be found on top of or in close proximity to a kimberlite body. Kimberlite is a type of volcanic rock that forms deep within the mantle of Earth and is a host to diamonds. Diopsides are unusual among igneous rocks because they contain mineral species with chemical compositions not generally found in most other igneous rocks, making them particularly useful as indicators for kimberlites and, therefore, diamonds.