Sphene
(Titanite)
The clear, green, yellow, or brownish stones provided by this species would be welcomed in jewellery because of their brilliant and almost adamantine lustre, but, unfortunately, they are too soft to withstand much wear, the hardness being only 5½ on Mohs’s scale. In composition sphene is a silico-titanate of calcium corresponding to the formula CaTiSiO5, and in this respect comes near the recently discovered gem-stone, benitoite. The refractive indices lie outside the range of the refractometer, the values of the least and the greatest of the refractive indices varying from 1·888 and 1·917 to 1·914 and 2·053 respectively. It is to this high refraction that it owes its brilliant lustre. The double refraction, which is biaxial in character and positive in sign, is so large that the apparent doubling of the opposite edges of a cut stone when viewed through one of the faces is obvious to the unaided eye (cf. [p. 41]). Cut stones have additional interest on account of the vivid dichroism displayed, the twin-tints, colourless, yellow, and reddish yellow, corresponding to the three principal optical directions, being in strong contrast. The specific gravity ranges from 3·35 to 3·45. The negative test with the refractometer (cf [p. 26]), the softness, and the large amount of double refraction suffice to distinguish this species from gem-stones of similar appearance.
The name sphene, from σφήν, wedge, alludes to the shape of the natural crystals. The alternative name is obviously due to the fact that the species contains titanium.
Good stones have come from the St. Gothard district, Switzerland.