The small green stones which accompany the diamond in South Africa have been cut and put on the market as ‘green garnet.’ They are, however, in no way connected with garnet, but belong to a mineral species called enstatite, which is a silicate of magnesium corresponding to the formula MgSiO_{3}; the green colour is due to a small amount of ferrous oxide which replaces magnesia. The double refraction is biaxial in character and positive in sign, the least and greatest of the refractive indices being 1·665 and 1·674 respectively; the specific gravity ranges from 3·10 to 3·13, and the hardness is only about 5½ on Mohs’s scale. The dichroism is perceptible, the twin-colours being yellowish and green, and, as usual, is more pronounced the deeper the colour of the stone. There is also a good cleavage in two different directions. With increasing percentage amount of iron enstatite passes into hypersthene. The colour becomes a dark brownish green, and an increase takes place in the physical constants, the least and greatest of the refractive indices attaining to 1·692 and 1·705 respectively, and the specific gravity ranging from 3·4 to 3·5. Hypersthene is never sufficiently transparent for faceting, but when spangled with small scales of brookite it is sometimes cut _en cabochon_. The name enstatite is derived from ἐνστάτης, an opponent, referring to the infusibility of the mineral before the blowpipe, and hypersthene comes from ὑπερσθένος, very tough. An altered enstatite, leek-green in colour and with nearly the composition of serpentine (p. 289), has been cut _en cabochon_. It has much lower specific gravity, only 2·6, and lower hardness, 3½ to 4 on Mohs’s scale. It is named bastite from Baste in the Harz Mountains, where it was first discovered. 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(SiO_{3})_{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.