Diopside is a monoclinic pyroxene mineral with composition MgCaSi₂O₆. It forms complete solid solution series with hedenbergite (FeCaSi₂O₆) and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull green crystals in the monoclinic prismatic class. It has two distinct prismatic cleavages at 87 and 93° typical of the pyroxene series. It has a Mohs hardness of six, a Vickers hardness of 7.7 GPa at a load of 0.98 N, and a specific gravity of 3.25 to 3.55. It is transparent to translucent with indices of refraction of n_α=1.663-1.699, n_β=1.671-1.705, and n_γ=1.693-1.728. The optic angle is 58° to 63°.