Spinel
Spinel is found in a wide range of colours due to the presence of various impurities, and is transparent to almost opaque. Red spinel coloured by chromium and iron is the most popular, although for many years it was thought to be a variety of ruby. The orange-yellow or orange-red is called rubicelle (a diminutive of the French word for ruby). Blue spinel is coloured by iron, and less commonly by cobalt. Occasionally, inclusions of crystals such as magnetite or apatite may occur, and some Sri Lankan spinels may include zircon crystals surrounded by brown haloes. Star stones are rare, but when cut en cabochon may show 4-rayed or 6-rayed stars.
Occurrence
Spinel occurs in granites and metamorphic rocks, and is often found in association with corundum. Octahedral crystals are waterworn pebbles in a wide range of colours are found in the gem gravels or Burma, Sri Lanka, and Madagascar. Other localities include Afghanistan, Pakistan, Brazil, Australia, Sweden, Italy, Turkey, the former USSR, and the USA.
Remark
Synthetic spinel has been manufactured since 1910. It has been used to imitate diamond, or coloured to imitate stones such as aquamarine or zircon. Blue synthetic spinel, coloured by cobalt, has been used to imitate sapphire. The name may derive from the Latin word spina, meaning little thorn, referring to the sharp points on some crystals.
| Crystal structure | Cubic |
| Composition | Magnesium aluminium silicate |
| Hardness | 8 |
| SG | 3.60 |
| Rl | 1.71-1.73 |
| DR | None |
| Lustre | Vitreous |
