Ĭorundum for abrasives is mined in Zimbabwe, Pakistan, Afghanistan, Russia, Sri Lanka, and India. The record has since been surpassed by certain synthetic boules. The largest documented single crystal of corundum measured about 65 cm × 40 cm × 40 cm (26 in × 16 in × 16 in), and weighed 152 kg (335 lb).
It commonly occurs as a detrital mineral in stream and beach sands because of its hardness and resistance to weathering. Other occurrences are as masses adjacent to ultramafic intrusives, associated with lamprophyre dikes and as large crystals in pegmatites.
It also occurs in low-silica igneous syenite and nepheline syenite intrusives. Geology and occurrence Corundum from Brazil, size about 2 cm × 3 cm (0.8 in × 1 in)Ĭorundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. In addition to its hardness, corundum has a density of 4.02 g/cm 3 (251 lb/cu ft), which is unusually high for a transparent mineral composed of the low- atomic mass elements aluminium and oxygen. It is a black granular form of corundum, in which the mineral is intimately mixed with magnetite, hematite, or hercynite. Emery, a variety of corundum with no value as a gemstone, is commonly used as an abrasive. It is commonly used as an abrasive on sandpaper and on large tools used in machining metals, plastics, and wood. īecause of corundum's hardness (pure corundum is defined to have 9.0 on the Mohs scale), it can scratch almost all other minerals. The name "corundum" is derived from the Tamil- Dravidian word kurundam (ruby-sapphire) (appearing in Sanskrit as kuruvinda). A rare type of sapphire, padparadscha sapphire, is pink-orange. Rubies are red due to the presence of chromium, and sapphires exhibit a range of colors depending on what transition metal is present. Corundum has two primary gem varieties: ruby and sapphire. It is a naturally transparent material, but can have different colors depending on the presence of transition metal impurities in its crystalline structure. May fluoresce or phosphoresce under UV lightīlack granular corundum intimately mixed with magnetite, hematite, or hercyniteĬorundum is a crystalline form of aluminium oxide ( Al 2O 3) typically containing traces of iron, titanium, vanadium and chromium. May alter to mica on surfaces causing a decrease in hardness Steep bipyramidal, tabular, prismatic, rhombohedral crystals, massive or granular CorundumĬolorless, gray, golden-brown, brown purple, pink to red, orange, yellow, green, blue, violet may be color zoned, asteriated mainly grey and brown Like all fine gemstones, the higher the carat or weight, the more valuable the price per carat is.Not to be confused with Carborundum. As always, the liveliness of the gemstone, often due to the way it was cut, is an important feature. As soon as the stone tips that scale, and become too saturated, the stone begins to lose value rapidly. Sri Lankan stones are notorious for having this beautiful raspberry-red color range!Īs with many colored gemstones, the greater the saturation of color, the more valuable the gemstone. The most sought after rhodolite garnets are the gemstones with raspberry hue with red body color. Rhodolites can be found in all variations of pink/red rosy-pink, raspberry-red, purple-pink, purplish red. Rhodolite Garnet Value | Color, Saturation and Carat Weight Generally cabochons are fashioned out of more included material and set into materials like sterling silver. Very rarely we have found the stone cut into cabochon form. When buying rhodolite garnet, it is almost always already cut and faceted.
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