Axinite
Called axinite from the shape of its crystals—ἀξίνη, axe—this species supplies small, clear, clove-brown, honey-yellow, and violet stones which can be cut for those who care for a stone out of the ordinary. The composition is a boro-silicate of aluminium and calcium, with varying amounts of iron and manganese, corresponding to the formula (Ca,Fe)3Al2(B.OH)Si4O15. Axinite is interesting on account of its strong dichroism, the twin-tints corresponding to the principal optical directions being violet, brown, and green. The double refraction is biaxial in character and negative in sign, the least and greatest of the refractive indices being 1·674 and 1·684; the specific gravity is 3·28, and hardness about 6½ to 7, or rather under that of quartz.
The best examples have been found at St. Cristophe, Bourg d’Oisans, in the Dauphiné, France. Violet axinite is a novelty that has come within recent years from Rosebery, Montagu County, Tasmania.