A sapphire is a variety of corundum, a very allochromatic mineral. It is the only substance which has a hardness of 9 on the Mohs scale of mineral hardness.Corundum is called sapphire when it is analuminium oxide. If the term “sapphire” is not preceded by an adjective that specifies the colour, it refers to the blue gem. However, there are natural sapphires of many different colours; such as yellow sapphire, pink sapphire or green sapphire. All of them are very valuable and used in the jewellery sector.The light, and the way through which it penetrates the stone, has a strong influence on the gem colour. As a matter of fact, sapphire is characterised by the phenomenon of double refraction. The ray of light, when crosses the stone, is split into two rays; for this reason, if we look at an object through the crystal, it will appear split. This phenomenon gives to the sapphire a sparkling and rich of shade colour. The blue colour of sapphire is due to the presence of rutile and hematite inclusions. These inclusions could appear in an intersection needlelike pattern, like a star. This phenomenon is called asterism and the stone that presents it is known as “star sapphire”. If there are only rutile inclusion the pattern will be a six-rays star; while, if the hematite will be present too, the star will have twelve rays, in particular, six whitish rays (the rutile ones) and six golden ones (as results of the hematite inclusions). The value of a star sapphire depends on the visibility and intensity of the asterism.Sapphires are naturally found by searching through metamorphic or magmatic rocks as well as through their alluvial deposits. The most important mines in the world are the ones in Sri Lanka (in particular, the gemstones from Ceylon are the most renowned), in Australia, in Burma and in Thailand. It is possible to find deposits of rubies in Italy too, on mount Terminillo.
The most used cuts for sapphires are the round and oval faceted ones; however is it possible to find very fine sapphires in heart or baguette shape. By contrast, the best cut for a star sapphire is the cabochon one. Thanks to this cut, it is possible to try to make the star visible on the top of the stone.
We still do not know the origin of the word “sapphire”; it could come from the Greek term sappheiros (light blue) or from the Hebrew sappir (the most beautiful thing). Anyway, has always been a beloved gemstone and its name brings to memory a specific shade of blue; Dante too, in the first canto of Purgatorio wrote “Sweet colour of oriental sapphire” to indicate the strong blue of the sky just before the dawn.
It was Auguste Verneuil in 1902 who invented the first synthesis process to reproduce sapphire crystals. Today, there are five differ methods to create a synthetic sapphire. It is possible to distinguish a natural sapphire to a synthetic one observing the internal inclusion with the help of a microscope and through the spectrometric analysis.The synthetic sapphire is used to produce the watches front screen protectors. In fact, sapphire has a hardness of 9 on the Mohs scale of mineral hardness, therefore is almost impossible to scratch.
The most famous sapphire in the World is the “Star of India”, it is a star sapphire and it weights 563 carats. It is currently on display at the American Museum of Natural History in New York. It is the biggest blue sapphire but not the star sapphire with the absolute highest weight. This record is achieved, with its 733 carats, by the “Black Star of Queensland”. It is owned by a private collector.The blue sapphire is the birthstone of September and the stone used to celebrate the forty-fifth wedding anniversary.