Category  :  Magnesium minerals, Calcium minerals, Silicate minerals
Hardness  :  5.5 - 6.5
Gravity  :  3.25 to 3.55
Reflection  :  1.72 / 1.69
Color  :  Green
Chemical formula  :  MgCaSi2O6
Crystal system  :  Monoclinic
Major varieties  :  Diopside, Star Diopside

Diopside is found in ultramafic (kimberlite and peridotite) igneous rocks, and diopside-rich augite is common in mafic rocks, such as olivine basalt and andesite. Diopside is also found in a variety of metamorphic rocks, such as in contact metamorphosed skarns developed from high silica dolomites. It is an important mineral in the Earth's mantle and is common in peridotite xenoliths erupted in kimberlite and alkali basalt.

Diopside is a precursor of chrysotile (white asbestos) by hydrothermal alteration and magmatic differentiation; it can react with hydrous solutions of magnesium and chlorine to yield chrysotile by heating at 600°C for three days. Some vermiculite deposits, most notably those in Libby, Montana, are contaminated with chrysotile (as well as other forms of asbestos) that formed from diopside.

At relatively high temperatures, there is a miscibility gap between diopside and pigeonite, and at lower temperatures, between diopside and orthopyroxene. The calcium/(calcium+magnesium+iron) ratio in diopside that formed with one of these other two pyroxenes is particularly sensitive to temperature above 900°C, and compositions of diopside in peridotite xenoliths have been important in reconstructions of temperatures in the Earth's mantle.

Star diopside

This is generally a blackish or blackish green color; rarely a definite green. It is, of course, cut into round or oval cabochons, which are generally biconvex with a roughly shaped, unpolished base. The star characteristically has 4 rays, two of which are straight, while the other two, not at right angles to the first pair, look slightly wavy.

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