The lovely velvety blue-purple variety of zoisite only comes from one small region of Tanzania called the Merelani Hills near Kilimanjaro. Only discovered in 1967 by a Masai tailor and part time prospector, it was first promoted in the gem trade as Tanzanite by Tiffany’s. It is strongly pleochroic, an attribute of some gems that split light rays entering them that then travel along different axes within the crystal related to its structure. The best specimens exhibit blue, violet and burgundy red when viewed from different directions (see http://bit.ly/2w6bdr8).
A hydrated calcium aluminium silicate, it was formed during the metamorphic event of the Pan African orogeny as Gondwana assembled, discussed in a previous post at http://bit.ly/1D548m8. Most specimens when mined have a yellow brown colour, which is removed by stable low temperature heat treatment, allowing the stunning violets to emerge. The impurity causing the hues remains unclear. Heating removes the red component, so the specimen in the photo is clearly unroasted. The first ones found were blue, having been heated in a lightning fire, demonstrating the effect of serendipity in all prospecting. The blues emerge more strongly in fluorescent lights, and the violets in the now rare incandescent bulbs. Occasional green stones also emerge, though they are strictly speaking green zoisite and not its beautiful blue cousin.
It is a fairly soft gemstone (Mohs hardness 6.5), usually used in pendants rather than rings, and care should be taken not to scratch it. Mining is both artisanal and mechanised, with Tanzanite One operating one of the four blocks as a larger scale integrated mine and the others being exploited by traditional methods divided into claims. Ethically sourced Tanzanite is possible, and the government have banned exporting unprocessed crystals to India in order to develop the indigenous faceting industry and to retain jobs and value added within the country. The whopper in the photo measures 10 x 6.0 x 4.5 cm, is unheated (it displays all 3 pleochroic colours, the red would be removed by the heat) and has a deep well saturated blue.
Image credit: Rob Lavinsky/iRocks.com