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Definition: spinel from The Columbia Encyclopedia

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.


Summary Article: Spinel from Guide to Gems

For centuries, spinels have been mistaken for RUBIES. For example, the so-called ‘Black Prince's Ruby’ in the British Imperial state crown is actually a 170-carat spinel. Until 1851, the ‘Kuwait Ruby’, also part of the British Crown Jewels, was considered to be the world's largest ruby, weighing 352.5 carats. It is relatively easy to mistake spinel for ruby because the two gems share many of the same desirable properties: they are chemically similar, deriving their red hue from chromium, and they have similar lustre, density, and hardness. Like rubies, spinels seem to fluoresce or glow in natural daylight. Spinels tend, however, to have a slightly pinker hue than rubies.

The derivation of the name is uncertain. It may come from the Latin spina for ‘spine’ or ‘thorn', because the stone is often found as sharp crystals. Since medieval times, it has also been known as the ‘Balas ruby’, after Balascia (today Badakhshan), a region of northeast Afghanistan that for many years was a source of fine specimens of spinel.

Spinel occurs as an accessory mineral in basic igneous rocks. Spinel's hardness and resistance to weathering means that it can be found as rolled pebbles in river and beach sands. It is often found in association with rubies and SAPPHIRES. Gem-quality spinels occur in limestone or limestone gravel and are most commonly found in Burma (Myanmar), Sri Lanka, India, and Russia. Undoubtedly, the finest spinel comes from the Mogok mines of Mandalay, central Burma.

Although it is red spinels that are the most renowned, the stone occurs in a variety of colours from hot reds and pinks through to cooler blues and greens and even a black, non-translucent variety called ceylonite. The reddest stones are known as ruby spinels, orange-reds are known as rubicelles, and green are sometimes called chlorospinels.

Rubies may be more prized – and more expensive – but good, gem-quality spinels are more rare. The best spinels are chosen for their lively colour. They are cut into brilliants, cushions, step cuts and are occasionally fashioned as round cabochons. Spinels can also be synthesized and synthetic spinels are used to imitate other gems. The magnetic properties of the spinel mineral magnetite, also known as lodestone, was used to help ancient mariners with navigation.

Scissors-cut blue spinel

© 2003 Philip's

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