A very interesting gemstone that is not commonly seen is spinel, a red gemstone often confused with ruby. Indeed, until fairly recently all red gemstones were called rubies; however ruby is denser and slightly harder than spinel, (spinel is 8 on the Mohs scale) and the two can be easily differentiated by comparing refractive indices.
Spinel is a hard magnesium aluminium oxide that has been used as a gemstone for centuries. It is known to come in a range of colours, from a rose pink to a rich red, and also lavender, deep violet, light and deep blue, orange, yellow, brown and black. Below is a rough piece of spinel.
The name spinel is thought to have come from the Greek word for “spark”, in reference to its bright colour. One of the most famous spinels of all is the 170 carat “Black Prince’s Ruby”, set into the British Imperial State Crown (seen below), above the 317 carat Cullinan II diamond. It was acquired by Edward, Prince of Wales, in 1367. His nickname was the Black Prince, from which the name of the stone was taken.
Spinel occurs in Cambodia, Myanmar, Sri Lanka and Thailand. There are some other locations where spinel deposits have been found, such as Afghanistan, Australia, Brazil, Madagascar and the USA.
The most desirable spinel colour is vivid red, followed by cobalt blue, bright pink and bright orange. Paler colours such as lavender tend to be less valuable. Other colours are black, violet-blue, greenish-blue, greyish, pale-pink, mauve, yellow and brown.
Good quality spinel gemstones should have good clarity, with no visible inclusions. Inclusions typically decrease the value of spinel except in rare circumstances where they can result in asterism (the star effect).
Other famous spinels in the world include
The Samarian Spinel, a red 500 carat gemstone that is part of the Iranian Crown Jewels
The Mogul Names Necklace, comprised of eleven red spinel gemstones with a total carat weight of 1,131.59 carats. It sold for over five million dollars at Christie’s in 2011.
A red 398 carat spinel set into the Russian Imperial Crown
The 352 carat “Timur Ruby”. It was believed to be the largest ruby until 1851, until is was discovered to be a spinel.