Knowledge

Sphene

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 Sphene, which is a either yellow, brown or green in gem quality mineral, is a very brilliant transparent stone with a resinous to adamantine lusture. It is named after a wedge-like shape of the monoclinic crystals, which are often twinned. The crystals are found in cavities in gneiss and granite, and also in such metamorphic rocks as schists and certain granular limestones.

 

Cut sphenes are characterised by having a pronounced fire, hence for the best effect the stone are always facated in brilliant or mixed cut styles and never in the cabochon form. Sphenes are magnificent stones when freshly cut, but the low hardness of 5.5 on Moh's scale does not make the stone suitable for constant wear.

 

Sphene is a silicate of titanium and calcuim (CaTiSiO5) and it is from the titanium content that the alternate name for the species, titanite, is derived. The latter name, however is more usually applied to the black or reddish-brown non-gem quality material. Some iron is always present in sphene and the rare-earth cerium and yttrium are commonly present. This trace of rare-earth impurity manifests itself in the absorption spectrum which generally shows, albeit weakly, the typical rare-earth spectrum of didymium.

 

The refraction of sphene is biaxial and positive in sign, the refractive indices varying from1.885 and 1.990 and 2.050 for the two major directions. The double refraction likewise varies from 0.105 to 0.135, and thus exceeds in amount many other gemstones, so that the doubling of the back facets will be observed with ease. The stones show strong trichroism, the colors for the principal directions being greenish-yellow, reddish-yellow and nearly colorless, but the exact shades depend on the color of the stone. The dispersion of sphene is 0.051, and greater than that of diamond. The density varies betwenn 3.52 and 3.54 and sphene , probably owing to the presence of iron, does not exhibit luminescence. The stones are brittle and also have a weak cleavage.

 

The major localities for gem sphene are at Schwarzenstein and Rotenkopf in the Zillerthal of the Austrian Tyrol, in the St. Gotthard and the Grisons in Switzerland, in Renfrew County, Ontario, Canada, in Madagascar, and a very fine cut stone of 22.26 carats, which had a density of 3.542, and was an orange-brown color came from the Burma Stone Tract. Both brown and green colored sphenes are found at El Alamo, Pino Solo and San Quintin in Baja California. Brazil is another source.

 

Source - GEMS - Their source, descriptions and identification. 3rd Edition - R. Webster