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http://www.gemmineonline.com/about.html (you will have to type the link on the web to see this.)12/20/2015
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Most gemstones are formed in earth's crust, but peridot is formed much deeper in the mantle region. Peridot crystals form in magma from the upper mantle and are brought to the surface by tectonic or volcanic activity where they are found in extrusive igneous rocks. Historically the volcanic island Zabargad (St. John) in the Red Sea was the location of the most important deposit. It was exploited for 3500 years before it was abandoned for many centuries; later, it was rediscovered around 1900 and has been heavily exploited ever since.   Tanzanite is relatively new on the gemstone market, but has left its mark. Its blue-lavender color is rather unique and a wonderful addition to the gemstone palette. Found in Tanzania (hence the name) in 1967, it has since become a well known and widely distributed gemstone. It has become so popular that in October of 2002 the American Gem Trade Association (AGTA) announced that tanzanite had joined zircon and turquoise in the traditional list of birthstones for the month of December.
It has better fire than the tourmaline elbaite or peridot and an adequate hardness. Its only one direction of cleavage is somewhat of a problem because it is oriented with the direction of strongest pleochroism. This would be a problem in most gemstones because that is the direction the gemcutter would usually select to maximize the color. However, with tanzanite the color is usually strong enough anyway. Pleochroism is very pronounced in tanzanite and is seen as three different color shades in the same stone. In the viewing a tanzanite stone, the colors dark blue, green-yellow and red-purple can be seen, all a result of pleochroism. Lesser stones may have a brownish color due to the mixing of blue, purple and green. These stones are usually heat treated to a deep blue color. Iolite is a blue-violet gemstone variety of the mineral cordierite, has strong pleochroism and can be confused with tanzanite. However, iolite is usually less strongly colored, its pleochroic colors vary from blue-violet to yellowish gray to blue and it has less fire. Iolite's unusual color shades makes it an exotic colored gemstone whose popularity is growing day by day. Nearly all tanzanite has been heat treated to generate the beautiful violet-blue color this stone is known for. When first mined, most stones are a muted green color. The only known source of Tanzanite is a five square mile hilltop at Merelani, ten miles south of the Kilimanjaro International Airport in Tanzania.  Drusy (or druzy) is a geological term applied to quartz that forms a layer of closely spaced, small crystals that line a surface or cavity of another kind of rock. Drusy quartz, silicon dioxide, is most commonly clear or whitish, and can resemble glittering sugar or snow crystals. It occurs within geodes and lines the walls of cavities called vugs that occur within hollows and veins of rocks.
Formation
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