Vesuvianite

Vesuvianite forms in skarns and other contact-metamorphosed limestones and dolomites, in serpentinites and related rocks, and in some alkalic and regionally metamorphosed rocks. The type locality is Monte Somma (Vesuvius), Italy. Cuttable material comes from Italy (brown and green), Quebec in Canada (pale green, bright yellow), Pakistan (green), Kenya (brown and green) and New York; other noted sources include Norway (blue cyprine), the Vilyui River in Russia (wiluite), Mexico and California, where californite, a massive vesuvianite-grossular mixture, was first reported.

Vesuvianite closely resembles grossular garnet, with which it is often intergrown, and its properties vary widely with its complex chemistry. Useful clues are its refractive index (commonly about 1.70 to 1.72) with very low birefringence, a uniaxial optic character, and a strong absorption line near 461 nm. Massive green californite can pass for jade; its density (3.25 to 3.32) and the 461 nm band separate it from jadeite, which shows a chromium spectrum. Idocrase and vesuvianite are the same mineral, not two different stones.

The mineral was described in 1723 by Moritz Anton Kappeler as "hyacinthus dictus octodecahedricus" and called "hyacinte du Vésuve" by Romé de l'Isle in 1772. Abraham Gottlob Werner named it "vesuvian" in 1795 after Mount Vesuvius, where it occurs as small, perfect crystals. In 1799 René Just Haüy introduced the name "idocrase", from Greek words meaning "mixed form", because its crystal forms resemble those of other species.