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Jewelry News09/05/2008 09/05/2008 Refer a FriendStore Information |
Peridot
 Peridot (pronounced peridow), a transparent green gemstone popular since antiquity, presents a problem in nomenclature. In America it goes by the name of chrisolite, but 'chrysolithos' meant a 'golden stone' to the ancients who used the term to describe what we now call yellow topaz; 'topazios', contrarily, referred to peridot. European mineralogists call the stone 'olivine' but this term (like chrysolite) includes both gem and non-gem qualities; more over, it has been wrongly used demantoid garnet. Peridot is a member of an isomorphus series of minerals, halfway between a magnesium silicate, forsterite, and and iron silicate, fayalite. It therefore contains both iron and magnesium in proportions that vary a little from stone to stone. Since iron also gives the gem its idiochromatic color, the depth of this color differs between specimens , but is nearly always green. Khaki and brownish stones occur as rarities but are noe particularly attractive. Stones of lustrous brown and yellow shades, until 1951 taken to be peridots, were identified in that year as belonging to a new and completely different mineral species, sinhalie. All peridots have a strong bire fringence which gives them a peculiar oily lustre and is an important clue in their identification. Stones of the best quality usually have a hardness nearer 6 than 7 and such stones should therefore not be exposed to hard wear. Peridot doesn't fluoresce but displays a distinctive absorption spectrum.  Peridot forms at very high temperatures both in igneous and metamorphic rocks; it is therefore found in a variety of environments. Crystals up to the size of pea have been found embedded in meteorites. It is the only gemstone so far reported to have been found in Antarctica. The most famous source of gem material, probably mined since the days of ancient Egypt and Rome, is the island of Zeberged (ST John's) of the coast of Egypt in the Red Sea. This may well be the island mentioned by Pliny as the source of the green 'topazios', famous in antiquity and one of the stones of the Jewish High Priest's breast-plate. Peridot of good quality came from there until the mines were closed down in 1914. Stones of even better quality come from the Mogok district in upper Burma, though the output is restricted at present because of bandit troubles. As may be inferred from the name, peridots are found in the diamondiferous periodotite pipes in South Africa, but these tend to be small. Gem-quality stones are mined in Minas Gerais, Brasil, while paler stones are found in Norway and darker stones in the gem gravels of Ceylon. Peridot is also found in Queensland, Australia and Arisona. Peridot is not particularly brilliant and the most appropriate cuts for it are the step cut or the scissors cut. It is set in necklaces, broches, earrings and other jewels and looks charming surrounded by small amethysts, rubies or garnets. It has been widely used in regalia and reliquaries, where it often masquerades as emerald. Famous treasures in which peridots are incorporated are on display at the Cathedral of Cologne in Germany and in the Armoury of the Kremlin, Moscow. Imitations exist in glass doublets, synthetic spinel and synthetic corundum but these rarely attain the right color and never the strong birefringence so characteristic of peridot. 
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