(The 'Black Prince's ruby" atop the Koh-i-Noor diamond in the Imperial State Crown is actually spinel)
Until fairly recently spinel was a largely unrecognised gem in relatively low demand. Lately the growth of coloured gems, including rubies, has lead to a renaissance in the appreciation for this richly coloured gem and its fascinating history. In antiquity the gem mines of southeast Asia produced exceptionally large spinel crystals, many of which found their way into treasuries of royalty and warlords alike from ancient Rome to China. Spinel closely resembles ruby in appearance and has often been mistaken for such, so much so that the finest spinel have sometimes been known as Balas 'rubies.' In many cases some of the finest 'rubies' in many royal courts were actually spinel. One of the most well documented examples of this was the "Black Prince's ruby." This crimson-red gem is set into the United Kingdom's Imperial State Crown and it is kept with the Crown Jewels at the Tower of London. The octagonal gem was most likely mined in the mountains of Afghanistan. It first appeared in Spain in the 1300s and found its way into the possession of several Spanish and Moorish monarchs before Edward, Prince of Wales (the "Black Prince") received the stone as a spoil from a military victory in 1367. Since this time it has adorned every English, and later British, head of state to the present day, surviving the English Civil War, the attempted theft of Colonel Blood and the Blitz to take pride of place alongside the Koh-i-Noor diamond at the centre of the Crown Jewels.
(The Timur ruby, set into a necklace, is another famous spinel)
The 'Timur ruby' weighs in at an impressive 350 carats and its history is just as intriguing as its illustrious cousin the 'Black Prince's ruby,' with several ancient Persian inscriptions carved into its surface to bear testament to its grand age. Until the 1800s spinel mined from Afghanistan and Tajikistan were commonly referred to as 'rubies' due to their deep red colouration. To this day both red and pink spinel are still produced in these areas but Sri Lanka and Myanmar (Burma) are now the primary source of gem quality spinel, including the rare cobalt blue stones. In terms of quality, the finest stones are produced in Burma and unlike ruby and jadeite, spinel are not subject to US trade sanctions on Myanmar. Burmese 'blood' red spinel are the closest to the much coveted 'pigeon's blood' red Burmese rubies of the same region and akin to garnet and diamond spinel is singly refractive with the same physical properties in all crystal directions. Spinel belongs to the cubic crystal system of stones and it is usually octahedron in shape, often appearing as if it were two pyramid shapes joined together at the base. Spinel can sometimes form flattened crystals, which look very different from their octahedral counterparts, caused by the pyramids that form the octahedron rotating against each other during their formation, creating a so called 'twinned crystal.'
(These are just some of the varieties of spinel)
Spinel can be found in a huge range of colours, hues and tones, from deep red to vivid pink, orange, purple, blue, violet and even bluish green. The intense reds and pinks are usually created by the presence of trances of chromium in the crystalline structure and the higher the chromium content, the deeper the red hue. Orange and purple spinel are created by a combination of iron and chromium, whereas violet and blue stones can be accounted for by traces of iron and cobalt. Red and pink spinel command the highest prices but clarity and carat do hold some sway and Burmese provenance is also a value multiplier. As with most coloured gemstones, when buying spinel it is important to remember that it is the spectral purity (or saturation) of the colour that has the most profound effect upon the a stone's value. Interestingly although peak quality red spinel is actually rarer than top priced rubies, they are in fact not quite as expensive! Orangey-red stones are referred to as 'flame' spinels and pink varieties are sometimes called 'bubblegum.' Some extremely rare and highly prized spinel change colour from grayish blue in daylight to an amethystine colour when subjected to incandescent light. All spinel are Type II gems, indicating that they are usually included, and eye-clean examples will attract a premium. In common with other coloured gems, mild inclusions do not substantially detract from a stone's value.
(A very rare 'star' spinel, displaying its 6 pointed asterism)
As with corundum (rubies & sapphires) some extremely unusual examples contain inclusions that create an asterism or double asterism and these are referred to as 'star' spinel. Spinel can be cut into a vast array of faceted shapes to best display their alluring colours, the most popular of which include cushion, emerald, round and ovals, and star spinel are often fashioned into cabochon shapes. Fine red spinel are very rare in sizes over 3 carats and prices increase significantly at 1 carat and above. Some blue stones are available over 4 carats and some orange at over 5 for a premium. As black diamonds have increased in popularity in recent times, so have the relatively uncommon black spinel. Black spinel makes for a less expensive alternative to black diamonds and a more robust, although more expensive, substitute for black onyx. For the most part purchasers of spinel can take it for granted that the natural variety is rarely treated or enhanced. Synthetic spinel, however, are commonly found so it is vital to purchase a stone that is accompanied by a certificate from a reputable laboratory. Colourless spinel are almost non-existent in their natural state so almost all examples are lab-grown and a high proportion of yellow and green stones also turn out to be synthetic. The highly sort after red and pink stones can be created in a lab by the 'flame-fusion' method but a qualified gemmologist can discern a natural spinel by observation of its absorption spectrum through a spectroscope.
(A pink spinel & diamond ring by Tiffany & Co)