Sphene, also known as Titanite
Forming delightful intense green to intense yellow wedge shaped crystals (and occasional pinks or reds), this lovely mineral is a mainstay of collections, and often faceted for amateurs in order to display it’s fire (intense colour flashes as the stone is turned), which is more powerful than diamond’s. The phenomenon results from the fact that the different wavelengths of colour in white light have different energies, and are bent in varying amounts by the crystal structure as the light rays pass through. The first and traditional name comes from the Greek for wedge, while the other mineralogical one is more recent and comes from its high titanium content. The more intense greens are coloured by chromium, the reddish brown Greenovite variety by manganese, while the intense yellow-greens get their hue from rare earth elements, didymium in particular.
It is common in volcanic and medium to high grade metamorphic rocks (ie those that have been more seriously squeezed and baked in the depths of the crust). It is characteristic of mountains, and especially contact zones between different kinds of rock where chemical mixing has occurred during metamorphism, though it also occurs in hydrothermal veins or altered granites on occasion. It was named by one of the French pioneers of mineralogy, the abbot Hauy in 1795, though the relevant commission changed the official name in 1982. Well known localities include the Gilgit region of Pakistan, Badakhshan in Afghanistan, Vesuvio in Italy, India, Canada, Brazil, Russia and California, with most facet grade material coming from Madagascar.
It remains too soft for jewellery use, despite its spectacular appearance being a mere 5.5 on Mohs scale. Specimens with strong colours also display marked changes in hue with orientation a property known as pleochroism (see http://bit.ly/1DWqeoofor an explanation). It also has strong double refraction, cause by the same splitting of light rays as in pleochroism, which means that facets ir crystal edges will seem doubled when viewed through the stone. Heating stones also changes the colour to orangey red and such treated pieces should be declared down the supply chain to the point of sale. The 4.1 x 3.3 x 2.8 cm specimen was mined in Pakistan and sits on a bright marble matrix.
Image credit: Rob Lavinsky/iRocks.com