Spinel

Specific Gravity: 3.60
Refractive Index: 1.71-1.73
Birefringence (DR): None
Luster: Vitreous
Crystal Structure: Cubic
Composition: Magnesium Chromium Oxide
Hardness: 8

Spinel is found in a wide range of colors. That is due to the presence of various impurities and is transparent to almost opaque. Red spinel colored by chromium and iron is the most popular. It is interesting to know that for many years, it was thought to be a variety of ruby. The orange-yellow to orange-red variety is called rubicelle (a diminutive for the French word of ruby). The blood red spinel is sometimes known as ruby spinel.

Blue spinel is colored by iron, and less commonly, by Cobalt. Occasionally, inclusions of crystals such as magnetite and apatite may occur. Some Sri Lankan spinels may contain zircon crystals surrounded by brown haloes. Star stones (asterism) are rare, but when cut en cabochon, may show 4-rayed or 6-rayed stars.

Blue spinel containing zinc is called gahnospinel. It is named after the Swedish chemist, J.G. Gahn.

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Synthetic Spinel

Synthetic spinel has been manufactured since 1910. It has been used to imitate diamond, or colored to imitate stones such as aquamarine and zircon. Blue synthetic spinel, colored by cobalt, has been used to imitate sapphire. The name may derive from the Latin word spina, meaning little thorn. It refers to the sharp points in some crystals.

Occurrence

Spinel gem occurs in granites and metamorphic rocks, and is often found in association with corundum. Octahedral crystals and waterworn pebbles in a wide range of colors are found in the gem gravels of Myanmar (Burma), Sri Lanka, and Madagascar. Other spinel gem sources include Pakistan, Afghanistan, Brazil, Italy, Turkey, Russia, Sweden, Australia, the USA, and the former USSR.