spinel

spinel, magnesium aluminum oxide, MgAl₂O₄, a mineral crystallizing in the isometric system, usually as octahedrons. It occurs as an accessory mineral in basic igneous rocks, in aluminum-rich metamorphic rocks, and in contact-metamorphosed limestones. Common spinel usually ranges in color from dark green to brown or black, but transparent red, blue, and green varieties are found and are used as gemstones; Myanmar and Sri Lanka are the principal producing countries. Much gem-quality spinel is now produced synthetically.

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spinel

Mineral composed of magnesium aluminum oxide (MgAl₂O₄). Also called magnesia spinel, its colour, due to various impurities, ranges from blood-red to blue, green, brown, and colourless. Spinel is found in basic igneous rocks, granite pegmatites, and contact metamorphic limestone deposits, often in association with corundum. Synthetic spinel has been manufactured since the early 20th century for use as imitation gem stones. Spinel may also refer more broadly to any of various mineral oxides of magnesium, iron, zinc, or manganese in combination with aluminum, chromium, or iron.