Red Spinel

Spinel is magnesium aluminium oxide, and red spinel is coloured, like ruby, by chromium; because it occurs alongside ruby in the same marble-hosted deposits of Burma and central Asia and shows a very similar red, it was for centuries not distinguished from ruby at all. Several of the most famous rubies in the world are in fact spinels: the Black Prince's Ruby and the Timur Ruby in the British Crown Jewels are both red spinels.

The distinction was only established as mineralogy matured, and the recognition somewhat unfairly demoted the stone in the market despite its beauty and its hardness of 8. Spinel is singly refractive and often remarkably clean. It has little independent folklore, its history being folded into that of ruby; modern birthstone lists have added spinel as an alternate for August. Its metaphysical associations are recent and largely inherited from ruby by association.

Two of the most famous red stones in Europe are spinels, and both are still called rubies in the names they have carried for centuries.

The Black Prince's Ruby, set at the front of the Imperial State Crown of the United Kingdom, is a large irregular red spinel, pierced through as an Asian bead is pierced and with the hole plugged by a small ruby. The Timur Ruby, also in the Royal Collection, is another spinel, inscribed with the names of the rulers who owned it. Neither is corundum, and neither name is going to change, because the object's history is attached to the name.

The confusion was not an error at the time. Spinel and ruby occur in the same deposits, they can be the same colour, and their hardness is close enough that no simple test of the period separated them. Both are found in the same alluvial gravels of Burma, Afghanistan and Sri Lanka. The two were recognised as different minerals only in the later eighteenth century, once crystal form and chemistry were being examined systematically: ruby is aluminium oxide and forms hexagonal crystals, spinel is a magnesium aluminium oxide and forms octahedra.

The lesson is a general one for reading old inventories. A ruby in a medieval or early modern document is a red stone of the right hardness, not a mineral species, and the same is true of sapphire, topaz and emerald in the same sources. The atlas records both the historical name and the modern identification wherever the two diverge.