The Black Prince's 'Ruby': The 170-Carat Spinel That Isn't a Ruby
It sits in the frontal cross of the Imperial State Crown, and it is not a ruby at all but a 170-carat red spinel. The legends of 1367 and Agincourt are reported history; the mineralogy is certain.
The Black Prince's Ruby is one of the most famous stones in the Crown Jewels, and it is not a ruby. It is a red spinel of about 170 carats, set in the frontal cross of the Imperial State Crown. For centuries every red stone was called a ruby; mineralogy arrived later than monarchy.
Colour is not identity. That is the founding lesson of gemmology.
According to tradition, the stone was given to Edward of Woodstock, the Black Prince, in 1367 by Don Pedro the Cruel of Castile, and it is reported that Henry V wore it on his helmet at Agincourt in 1415. These are the stories the stone carries. They belong to the category of reported history rather than documented fact, and a serious publication says so.
Spinel, explained
Spinel and ruby are different minerals that can look identical to the naked eye. Both are aluminium oxides in the loose sense; ruby is corundum coloured by chromium, spinel is magnesium aluminium oxide. The distinction was not reliably made until the development of modern gemmology, which is why the great historic 'rubies', the Black Prince's stone and the Timur Ruby among them, are spinels.
The stone's fame does not depend on its species. A 170-carat red spinel with seven centuries of reported royal ownership needs no help from mineralogy. But the correction matters, because it is the founding lesson of gemmology: colour is not identity.
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