| CORUNDUM AND SAPPHIRE | ||
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Corundum (Al2O3) occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. It also occurs in low silica igneous syenite and nepheline syenite intrusives. Other occurrences are as masses adjacent to ultramafic intrusives, associated with lamprophyre dikes and as large crystals in pegmatites. Because of its hardness and resistance to weathering, it commonly occurs as a detrital mineral in stream and beach sands. Corundum for abrasives is mined in Zimbabwe, Russia, and India. kurundam ( from Tamil kuruvindam) is a crystalline form of aluminium oxide and one of the rock-forming minerals. It is naturally clear, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red, while all other colors are called sapphire. A pinkish-orange sapphire is called padparadscha. Due to corundum’s hardness (pure corundum is defined to have 9.0 Mohs), it can scratch almost every other mineral, leaving behind a streak of white on the other mineral. It is commonly used as an abrasive, on everything from sandpaper to large machines used in machining metals, plastics and wood. Some emery is a mix of corundum and other substances, and the mix is less abrasive, with a lower average Mohs hardness near 8.0. Sapphire refers to gem varieties of the mineral corundum, an aluminium oxide (Al2O3), when it is a color other than red. Sapphire can be found naturally or manufactured in large crystal boules. Because of its remarkable hardness sapphire is used in many applications, including infrared optical components, watch crystals, high-durability windows, and wafers for the deposition of semiconductors. The mineral corundum consists of pure aluminium oxide. Trace amounts of other elements such as iron, titanium and chromium give corundum their blue, yellow, pink, purple, orange or greenish color. Sapphire includes any gemstone quality varieties of the mineral corundum except the fully saturated red variety, which is instead known as ruby, and the pinkish-orange variety known as padparadscha. Sapphires are mined from alluvial deposits or from primary underground workings. The finest specimens are mined in Sri Lanka and the disputed territory of Kashmir. Both the Logan sapphire and the Star of Bombay originate from Sri Lankan mines. Sapphires are also mined in Australia, Madagascar, Thailand and Myanmar. Sapphires and rubies are formed at great depth in the earth’s mantle or the lowest part of the crust. Although blue is considered the normal color for sapphires, they can be found across a full range of spectral colors as well as brown, colorless, grey and black. Those other than blue in color are considered fancy color sapphires. Some natural sapphires can be found as completely transparent, or “white.” White sapphires usually come out of the ground as light grey or brown and are then heated to make them clear. However, in very rare circumstances they will be found in a clear state. |
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Blue sapphire Various shades of blue [dark and light] result from titanium and iron substitutions in the aluminium oxide crystal lattice. Some stones are not well saturated and show tones of grey. It is common practice to bake natural sapphires to improve or enhance color. This is usually done by heating the sapphires to temperatures of up to 1800 °C for several hours, or by heating in a nitrogen deficient atmosphere oven for seven days or more. On magnification, the silk due to included rutile needles are often visible. If the needles are unbroken, then the stone was not heated; if the silk is not visible then the stone was heated adequately. If the silk is partially broken, then a process known as low tube heat may have been used. Low tube heat is the process whereby the rough stone is heated to 1300 °C over charcoal for 20 to 30 minutes. This removes grey or brown in the stone and improves color saturation. |
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Fancy color sapphire Purple sapphires are lower in price than blue ones. These stones contain the trace element vanadium and come in a variety of shades. Yellow and green sapphires have traces of iron that gives them their color. Pink sapphires have a trace of the element chromium and the deeper the color pink the higher their monetary value as long as the color is going toward the red of rubies. Sapphires also occur in shades of orange and brown, and colorless sapphires are sometimes used as diamond substitutes in jewelry. Salmon-color padparadscha sapphires often fetch higher prices than many of even the finest blue sapphires. The word ‘padparadscha’ is Sinhalese for ‘lotus flower’. Recently many sapphires of this color have appeared on the market as a result of a new treatment method called “lattice diffusion”. |
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Color change sapphire Color shift sapphires are blue in outdoor light and purple under incandescent indoor light. Color changes may also be pink in daylight to greenish under fluorescent light. Some stones shift color well and others only partially, in that some stones go from blue to bluish purple. Such color-change sapphires are widely sold as “lab” or “synthetic” alexandrite, which is accurately called an alexandrite simulant (also called alexandrium) since the latter is actually a type of chrysoberyl—an entirely different substance whose pleochroism is different and much more pronounced than color-change corundum (sapphire). |
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Star sapphire A star sapphire is a type of sapphire that exhibits a star-like phenomenon known as asterism. Star sapphires contain intersecting needle-like inclusions (often the mineral rutile, a mineral composed primarily of titanium dioxide ) that cause the appearance of a six-rayed ‘star’-shaped pattern when viewed with a single overhead light source. The value of a star sapphire depends not only on the carat weight of the stone but also the body color, visibility and intensity of the asterism. |
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