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Sapphire Gemstone Information

Sapphire is the variety of species corundum, it occurs in all colours except red (red corundum is ruby). The name comes from the Greek word for blue 'sappheiros' and is the colour referred to when sapphire is mentioned by itself or written without any reference to colour. All other colours are termed 'fancy' where the actual colour appears as a prefix to sapphire, for example 'pink sapphire' The most desired blue sapphires are a richly saturated blue to violetish-blue colour, some are described as velvety blue. The colours range from the lighter 'Cornflower Blue' to the deeper 'Royal Blue', prime specimens are found in Kashmir, Myanmar (formerly Burma) and Sri Lanka. Larger stones are sort after by collectors and investors and occasionally appear in auctions selling at very high prices.

Padparadscha sapphires are the most prized of the fancy coloured stones, they have a range of pink-orange colours that resemble those of the lotus flower. Deposits of fine stones are found in Sri Lanka, where the name originates from the Sinhalese for lotus flower 'padma ranga' describing it's unique colours. Sapphires with similar padparadscha colours are found in Africa and Vietnam.

Sapphires cut en cabochon may display a star effect known as asterism, made possible by the presence of fine needle-like inclusions of rutile, known as silk. The fine silk inclusions run in parallel and intersect at 60/120 degrees from each other, having grown in the three directions of the trigonal crystal system. In order for asterism to be seen, the long rutile inclusions must be oriented in-line with the base of the stone as it is cut. Normally a six-rayed star will appear on the surface of the stone following the cut and polish. Rarely a twelve-rayed star will appear, when there are two types of inclusion present, the long thin rutile together with small flat inclusions of hematite. A corundum crystal that has formed with parallel growth features (twinning) may also display a twelve-rayed star, when cut correctly.

Sapphires that display asterism are commonly blue or black in colour and display fine star effects.

Source locations for sapphire include: Africa, Australia, Brazil, Cambodia, China, Myanmar, Sri Lanka, Thailand, Vietnam and the USA.

Physical Properties

Stone type:
Sapphire is a variety of the species corundum.
Crystal System:
Trigonal.
Chemical Composition:
Aluminum oxide, Al2O3
Colour:
Sapphire occurs in all colours, except red, colourless corundum is white sapphire.
Lustre:
Bright vitreous.
Pleochroism:
Drichroic.
Dispersion:
Low.
Hardness:
9 on the Mohs' scale of hardness.
Toughness:
Excellent.
Cleavage:
Parting (splitting) may occur in stones with twin planes. These are stones where the intergrowth of crystals has taken place.
Density in gm/cc:
3.95 to 4.05
Double refraction:
Yes.
Refractive index:
1.76 to 1.78

Common Treatments

Most sapphires on the market have been heat-treated to improve the colour.
Blue stones are also produced by heat-treating very pale or colourless stones. Cloudy corundum called 'Geuda stones' in Sri Lanka, when heat-treated can produce spectacular blue stones.

Sapphires are also 'surface or lattice diffused', where a very thin layer of colour is diffused into the surface of pale coloured stones, by coating the stone in oxides and heating over a long period of time.
Chipped stones will reveal the original colour, re-polishing with abrasives will also remove the colour.

Beryllium diffusion of corundum is a fairly new concept in ruby and sapphire enhancement. The process diffuses beryllium into corundum to create yellow, orange, red, brown and blue colours from pale or nearly colourless material. It can also change an actual colour and transform dull coloured corundum into vibrant richly coloured material. The diffusion may be close to the surface or may penetrate the entire stone.


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