Idocrase, also known as vesuvianite, is occasionally found in the form of transparent, leaf-green, and yellowish-brown stones which, when cut, may be mistaken for diopside and epidote respectively, but are distinguishable from both by the extreme smallness of their double refraction. Californite is a compact variety which has all the appearances of a jade; its colour is green, or nearly colourless with green streaks. In composition idocrase is a silicate of aluminium and calcium, the precise formula of which is uncertain, but may be— (Ca,Mn,Mg,Fe)_{2}[(Al,Fe)(OH,F)]Si_{2}O_{7}. The double refraction, which is uniaxial in character and negative in sign, may be less than 0·001, and never exceeds 0·006, so that it is not easily detected with the refractometer, even in sodium light. The refractive indices vary enormously in value, from 1·702 to 1·726 for the ordinary, and from 1·706 to 1·732 for the extraordinary ray. The specific gravity varies from 3·35 to 3·45, and the hardness is about 6½ on Mohs’s scale. The name idocrase, from εἴδος, form, and κρᾶσις, mixture, was assigned to the species by Haüy, but his reasons have little meaning at the present day. The other names are taken from the localities where the species and the variety were first discovered. Bright, green crystals come from Russia, and also from Ala Valley, Piedmont, and Mount Vesuvius, Italy. Californite is found in large masses in Siskiyon and Fresno Counties, California. EPIDOTE (_Pistacite_) Epidote often possesses a peculiar shade of yellowish green, similar to that of the pistachio-nut—hence the origin of its alternative name—which is unique among minerals, though scarcely pleasing enough to recommend it to general taste. Its ready cleavage renders it liable to flaws; nevertheless, it is occasionally faceted. The name epidote, from ἐπίδοσις, increase, was given to it by Haüy, but not on very precise crystallographical grounds. In composition this species is a silicate of calcium and aluminium, with some ferric oxide in place of alumina, corresponding to the complex formula, Ca_{2}(Al,Fe)_{2}[(Al,Fe)OH](SiO_{4})_{3}. It occurs in monoclinic, prismatic crystals richly endowed with natural faces. The colour deepens with increase in the percentage amount of iron, and the stones become almost opaque. The double refraction is large in amount, 0·031, biaxial in character, and negative in sign. The dichroism is conspicuous in transparent stones, the twin-tints corresponding to the principal optical directions being green, brown, and yellow. The values of the least and greatest of the refractive indices given by transparent stones are 1·735 and 1·766 respectively; the specific gravity varies from 3·25 to 3·50, and the hardness from 6 to 7 on Mohs’s scale.