Sapphire
Sapphire is the variety of corundum that occurs in all colours except red (red corundum is ruby).
The name
is Greek for blue
and is
normally the colour referred to when sapphire is mentioned without reference to colour. Cornflour blue is the most prized blue colour, sort after by collectors and investors.
Padparadscha sapphires are the most prized of the fancy coloured stones, they are pink-orange in colour. Deposits are found in Sri Lanka, where the name originates from the Sinhalese for Lotus Flower. They are also found in Africa and Vietnam.
Material displaying asterism is cut en cabochon
to produce six and rarely 12 rayed stars. The stones that display asterism are commonly of blue or black material and display a fine star effect.
Source locations for sapphire include: Africa, Australia, Brazil, Cambodia, China, Myanmar, Sri Lanka, Thailand, USA and Vietnam.
Properties of the Gemstone and Additional Information
| Gemstone type: | .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 |
| 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. Re-polishing these stones will remove the colour.
Beryllium diffusion of corundum is a 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.