Diopside
This species, which is also known as malacolite and alalite, provides stones of a leaf-green colour which have occasionally been cut. It is a silicate of calcium and magnesium corresponding to the formula MgCa(SiO3)2, but usually contains in place of magnesia some ferrous oxide, to which it owes its colour; with increase in the percentage amount of iron the colour deepens and the physical constants change. The double refraction is large in amount, 0·028, biaxial in character, and positive in sign. The least and greatest of the refractive indices corresponding to the stones suitable for jewellery range about 1·671 and 1·699 respectively, but they may be as high as 1·732 and 1·750 in the two cases. The specific gravity varies from 3·20 to 3·38, and the hardness from 5 to 6 on Mohs’s scale. Dichroism is noticeable in deep-coloured stones, but is not very marked.
The name diopside comes from δίς, double, and ὄψις, appearance, in allusion to the effect resulting from the double refraction; malacolite is derived from μαλακός, soft, because the mineral is softer than the felspar associated with it; and alalite is named after the principal locality, Ala Valley, Piedmont, Italy.