Spinel

Spinel is a magnesium aluminum oxide mineral that has been used in jewelry for centuries. It occurs in a vast array of colors, with vibrant red being the most famous. Historically, red spinel was often mistaken for ruby, leading to it being called 'the great imposter' of the gemstone world. Many famous 'rubies' in royal crown jewels around the world are actually spinels.

Formation & Geology

Spinel typically forms in metamorphic rocks, especially those rich in magnesium and aluminum, such as marble. It is also found as a primary mineral in mafic igneous rocks and as rounded pebbles in alluvial gravels (placer deposits). It is often found in the same mines as ruby and sapphire.

Physical Characteristics

Spinel is quite hard (8 on the Mohs scale), making it very durable. It is isometric, which means it is singly refractive—this gives it a very clean, bright appearance because light isn't split into two rays. It often forms beautiful octahedral crystals that look like two pyramids joined at the base.

Historical Significance

The most famous spinel in the world is the 'Black Prince's Ruby' in the British Imperial State Crown—a 170-carat red spinel. Another is the 'Timur Ruby', also a spinel. It wasn't until 1783 that mineralogist Jean-Baptiste L. Romé de l'Isle identified spinel as a distinct mineral from ruby. It was recently added as a birthstone for August.

The Great Imposter

For centuries, spinel was confused with ruby and sapphire. The most famous example is the "Black Prince's Ruby," a 170-carat red spinel set in the Imperial State Crown of the United Kingdom. It was only in 1783 that mineralogist Jean-Baptiste Louis Romé de l'Isle distinguished spinel as a separate mineral. This history makes it a fascinating stone for collectors who appreciate its role in royal trickery.