Diopside is a monoclinic mineral with composition MgCaSi 2O 6. It forms complete solid solution series with hedenbergite (FeCaSi 2O 6) and augite , and partial solid solutions with orthopyroxene. 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 3.25 to 3.55. It is transparent to translucent with indices of refraction of nα=1.663–1.699, nβ=1.671–1.705, and nγ=1.693–1.728. The optic angle is 58° to 63°.