IDOCRASE, a rock-forming mineral of complex composition. It is a basic calcium and aluminium silicate containing small amounts of iron, magnesium, water, fluorine, etc., and sometimes boron; the approximate formula is . It crys tallizes in the tetragonal system, but often exhibits optical anom alies, and the optical sign varies from positive to negative. Well developed crystals are of frequent occurrence. They usually have the form of four- or eight-sided prisms terminated by the basal planes and pyramid-planes. Crystals are transparent to translucent, vitreous in lustre and vary in colour from brown to green; a sky-blue variety, called cyprine, owes its colour to the presence of a trace of copper. The specific gravity is 3.4 and the hardness 61. The name vesuvianite is also in common use for this mineral.
Idocrase is typically a mineral of contact-metamorphic origin, occurring most frequently in crystalline limestones at their con tact with igneous rock-masses; it also occurs in serpentine, chlo rite-schist and gneiss, and is usually associated with garnet, diop side, wollastonite, etc. Localities which have yielded fine crystal lized specimens are the Ala valley in Piedmont, Monte Somma (Vesuvius), the River Wilui in Siberia ("wiluite"), Christiansand in Norway, etc. When found as transparent crystals of a good green or brown colour it is occasionally cut as a gem-stone. A compact variety ("californite"), closely resembling jade in appear ance, has been used as an ornamental stone.