Sapphire is a gemstone formed of the mineral corundum, the same from which rubies are formed. The blue colour is caused by the corundum containing impurities of iron in the crystal structure.
Chromium is found in rubies giving them their red colouring.
It is the third hardest naturally occurring mineral behind diamonds (10) and moissanite (9.5) on the Mohs scale of hardness and weighs in at 9 on that scale. Predominantly used for jewellery, the stones also have uses in other fields. Mechanical watches may use sapphire bearings and have sapphire glass on the face of the watch as it is very scratch resistant. They also have uses in scientific instruments and optical devices.
Sapphires are typically blue, but other colours including pink and clear as well as various shades of yellow, grey and black also occur. The value of a blue sapphire is largely dependent upon the colour of their primary hue. Secondary hues including violet, purple and green will affect the value, with purple and violet secondary’s increasing value and green reducing the price. The best blue sapphires have a primary hue of blue of at least 85% and the secondary hues of violet/purple.
As with a diamond, the stones are rated using the 4C’s. Carat, colour, clarity and cut. One of the largest cut sapphires weighs 423 carats (84grams) named the Logan Sapphire which is displayed in the National Museum of Natural History in Washington DC.
Colours other than blue are also quite common, trace amounts of metallic impurities cause yellow (titanium), green (magnesium), orange (copper) and pink (chromium) stones to occur in nature. Pink sapphires have less chromium present than the amount needed to be judged a ruby.
Star sapphires display a 6 rayed star called an asterism. This is caused by inclusions (usually a mineral called rutile) that intersect beneath the top layer of the gemstone. These stones are generally cut as cabochons with the stars centre near the top of the stone. Twelve rayed stars can occasionally be found, usually the result of an additional set of inclusions. The Black Star of Queensland weighs 733 carats, being the largest known gem quality in the world.
A colour change sapphire can show different colours depending upon the type of light it is under. They will appear to be blue under sunlight and change to purple under incandescent lighting. These stones are quite rare and usually are found in Tanzania and Thailand.
To improve the clarity and colour of a sapphire, it may be treated with heat in a furnace of around 500 to 1800 degrees Celsius for several hours. This heat causes the stones to become more blue in colour and also causes the reduction of silk (rutile inclusions). Around 95% of all sapphires are heat treated for use in jewellery. A sapphire treated with too much heat, for too long, can lose all of its silk and become colourless.
Until the 1980’s it was not common knowledge that sapphires were heat treated at all. Yogo sapphires have a deep and consistent cornflower blue colour and generally don’t need to be treated. A company began marketing these stones as ‘guaranteed untreated’ which caused rifts between many in the gem industry.
Synthetic sapphires were first produced by Verneuil (a French chemist) in 1902. The process makes use of alumina powder and an oxygen/hydrogen flame. The flame is pointed downwards towards a mantle where the alumina from the flame is deposited over time. This creates a boule in the shape of a teardrop containing the sapphire crystals. By introducing various impurities, the colour of the resulting synthetic sapphire can be altered to match any of the colours that occur naturally.
The synthetically created sapphires are identical to those found in nature, however the flaws that are present in naturally occurring gemstones can be avoided. As flawless sapphires are rarely found naturally, this can be a strong indication that the stone is man made.
Sapphires have a wide range of uses for non-jewellery applications. Shatter resistant windows in armoured vehicles, barcode scanners in supermarkets, integrated circuits and lasers are just some of the ways in which this very hard and durable material is put to use.