Sapphire
All gem-quality corundum that is not red is called sapphire, yet the name is popularly associated with the blue. Variation in colour, due to the iron and titanium impurities, spans many shades, but the most valuable is clear, deep blue. Some stones, called “colour-change sapphire”, exhibit different shades of blue in artificial and natural light.
Occurrence
Good quality sapphire is found in Burma, Sri Lanka, and India. The best Indian sapphire is cornflower blue, and found in Kashmir, either in pegmatites or as waterworn pebbles in alluvial deposits. Sapphire from Thailand, Australia, and Nigeria is dark blue, and may appear nearly black. Montana (USA) produces sapphire of an attractive metallic blue. Other localities include Cambodia, Brazil, Kenya, Malawi, and Colombia.
Remark
Synthetic sapphire production began in the late 19th century. Commercial quantities became available in the early 20th century.
| Crystal structure | Trigonal |
| Composition | Aluminium oxide |
| Hardness | 9 |
| SG | 4.00 |
| Rl | 1.76-1.77 |
| DR | 0.0508 |
| Lustre | Vitreous |
Pink Sapphire
Pure pink sapphire is coloured by a very small amounts of chromium, and with increasing amounts of chromium it forms a continuous colour range with ruby. Tiny amounts of iron may produce pink-orange stones padparadscha, or iron and titanium impurities together may make a purple stone. Pink sapphires are often cut with a deep profile.
Occurrence
Spessartine occurs in granitic pegmatities and alluvial deposits. It is found in Sri Lanka, Madagascar, Brazil, Sweden, Australia, Burma, and the USA; also Germany and Italy, but crystals there are too small to facet.
Remark
Pink sapphires, from a very pale and delicate pink to a near-red, occur in Sri Lanka, Burma, and East Africa.
| Crystal structure | Trigonal |
| Composition | Aluminium oxide |
| Hardness | 9 |
| SG | 4.00 |
| Rl | 1.76-1.77 |
| DR | 0.008 |
| Lustre | Vitreous |
Padparadscha
Padparadscha is a very rare, pinkish orange sapphire. It is the only variety of corundum other than ruby that is given its own name, rather than being referred to as sapphire of a particular colour. The name derives from Sinhalese word, meaning lotus blossom.
Occurrence
Sri Lanka.
Remark
Like all varieties of corundum, padparadscha is an excellent jewellery stone, as it is second only to diamond in hardness.
| Crystal structure | Trigonal |
| Composition | Aluminium oxide |
| Hardness | 9 |
| SG | 4.00 |
| Rl | 1.76-1.77 |
| DR | 0.008 |
| Lustre | Vitreous |
Colourless Sapphire
The different colours found within members of the corundum group are due to small amounts of metal oxide impurities. Corundum without impurities (and therefore without colour) is rare, but when found is classified as colourless sapphire. Stones made up of different colours, including colourless areas, are more common. Stones like these are generally orientated by the cutter so that the colour is at the base. Then, when viewed from above, colour fills the stone.
Occurrence
Truly colourless sapphire is found in Sri Lanka. Cloudy or milky coloured sapphire is also found in Sri Lanka, and referred to locally as geuda. Heat treatment of geuda produces blue sapphire, much of which is faceted and used in jewellery. Some Sri Lankan corundum shows red, blue, and colourless areas, which may be faceted or polished to give an interesting stone.
Remark
Synthetic colourless corundum has been produced by the Verneuil method since about the 1920’s and has been called diamondite.
| Crystal structure | Trigonal |
| Composition | Aluminium oxide |
| Hardness | 9 |
| SG | 4.00 |
| Rl | 1.76-1.77 |
| DR | 0.008 |
| Lustre | Vitreous |
Green Sapphire
From medieval times until the end of the 19th century green sapphire was referred to as “oriental peridot”. Many sapphires that appear green actually consist of very fine alternating bands of blue and yellow sapphire, which may be visible under a microscope.
Occurrence
Green sapphires are found in Sri Lanka, Thailand, Australia (Queensland and New South Wales).
| Crystal structure | Trigonal |
| Composition | Aluminium oxide |
| Hardness | 9 |
| SG | 4.00 |
| Rl | 1.76-1.77 |
| DR | 0.008 |
| Lustre | Vitreous |
Yellow Sapphire
Up until the end of the 19th century yellow sapphire was known as “oriental topaz” (only blue corundum was called sapphire). Nevertheless yellow and greenish yellow sapphires make unusual and attractive gemstones in their own right.
Occurrence
In Queensland and New South Wales (Australia), a greenish yellow sapphire is found that may be faceted. Similar stones occur in Thailand, and purely yellow stones in Sri Lanka, Montana (USA), and East Africa.
| Crystal structure | Trigonal |
| Composition | Aluminium oxide |
| Hardness | 9 |
| SG | 4.00 |
| Rl | 1.76-1.77 |
| DR | 0.008 |
| Lustre | Vitreous |
