Topaz, with attendant tourmalineTopaz is a hydrated aluminium fluorosilicate, and a biggie in the ge

Topaz, with attendant tourmalineTopaz is a hydrated aluminium fluorosilicate, and a biggie in the gem world, with many beautiful varieties, some natural, others with artificially induced colours. Pure topaz is colourless, and fairly common in the right geological environment, but most such material ends up treated in the gem market (no problem, as long as the treatment is disclosed). Impurities in the crystal lattice can give it a variety of hues, from the sherry orangey red of the best known Imperial topaz crystal, through light blue, pink to violet (quite rare), yellow to golden brown and pale green.The traditional November birthstone is born in volcanic fire, when granites pond beneath the surface and crystallise, with the last hydrous remnants of the granite concentrating the incompatible elements (those that do not fit into the crystal structure of the common granitic minerals) such as the fluorine essential to this gem’s formation. It forms during the last stages of cooling in cavities in silica rich magmas, as pegmatitic crystals or from superheated fluorine rich gases gases (a process known as miarolitisation). Topaz also occurs in the eroded remnants of these rocks and as alluvial placer deposits, though the perfect basal cleavage (a line of weak bonds in the crystal structure along which the crystal can easily sit) perpendicular to the long axis of the crystal means that it cannot travel far before being battered to pieces (and that specimens on matrix are extraordinarily rare). The cleavage also makes faceting tricky, and every facet must be cut at an angle away from the cleavage direction or you’ll just lift chunks of the rough gem off with the lap.Natural blue topazes like the 3.8 x 3.4 x 3.2 cm crystal in the photo are very pale gems with low colour saturation. Any deep blue comes from irradiation and heat treatment (no health worries if done safely, and such material is routinely tested before release on the market), which can also strengthen yellows or induce the rare pink to purple hue. A good guideline is price, if it’ cheap it’s almost certainly treated. Due to the same factors that allow irradiation to work, some stones (natural and treated for example Siberian ones) can fade in colour on exposure to sunlight, though again most material in the jewellery market is tested for colour stability before sale. Another treatment, known as mystic topaz is similar to aqua aura quartz, and involves a coating to produce irridecence by thin film interference (just like oil on water)..The name comes from Latin via French, and is probably derived from the Greek island of Topazios (St John’s Island) in the Red Sea, though the Sanskrit word Tapas (fire) has also been suggested as the source of the derivation. For many centuries the name was applied indiscriminately to all yellow gems, along with names such as chrysolite (usually yellowish green peridot (see http://on.fb.me/1AwvQDF) or chrysoberyl(see http://on.fb.me/1XRqRer)) or hyacinth (red zircon, see http://on.fb.me/21zn33p). Topaz is the reference mineral for the Mohs hardness of 8, and hence hard enough for jewellery use, though care should be taken with the cleavage, avoiding thermal shock for example by putting a ring clad hand from the cold outside into an oven.Sources include the USA (including Topaz Mountain in Utah), the Ural Mountains of Russia (the classic light blue source), Nigeria, Afghanistan, Pakistan, the Mogok stone tract in Burma, Sri Lanka, China and especially the pegmatite fields of Brazil, from which the specimen in the photo hails (Ouro Preto in Minas Gerais). Colourless crystals can be the size of large boulders and some whopping facetted gens exist in museums and collections worldwide. Here the lovely pale Pakistani piece from the pegmatite fields of the Gilgit region of Baltistan has a rare addition in the form of a gemmy green to pink bicoloured tourmaline crystal (see http://bit.ly/2BMwdCs) that has grown alongside it. The combination is uncommon because boron rich pegmatites (for the tourmaline) are not often also rich in fluorine.LozImage credit: Rob Lavinsky/iRocks.comhttp://www.mindat.org/min-3996.htmlhttp://www.minerals.net/mineral/topaz.aspxhttp://geology.com/minerals/topaz.shtmlhttp://www.mineralminers.com/html/topminfo.htmhttp://www.gemdat.org/gem-3996.htmlhttp://bit.ly/1Qk074C -- source link

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