Sapphire
Corundum occurs in all colours. Sapphire is the name given to all colours of corundum other than red. When used on its own, the name implies blue sapphire. Blue Sapphire is coloured by a combination of iron and titanium. Some sapphires may exhibit a colour change.
Common treatments
Both heat treatment and diffusion treatment are common in sapphires.
Materials with a similar appearance
Blue glass, iolite, blue tourmaline, tanzanite, blue spinel, blue synthetic spinel and corundum-corundum doublet.
Localities and inclusions
| Locality | Comments and inclusions |
|---|---|
| Kashmir, India | Restricted production of fine-coloured blue sapphire, often with a very slight milkiness caused by very small inclusions; colour zoning crystals, stress fractures, negative crystals. |
| Myanmar (Burma) | Sapphires of good colour but sometimes rather dark. Inclusions may be long needles of rutile, apatite. Convoluted feathers, silk, hexagonal colour zoning present in some stones. |
| Thailand (siam) | Sapphire of poor to good colour. Straight and angular colour zones with silk, feldspar, hornblende, pyrrhotite, spinel, uranium pyrochlore and secondary fluid inclusions. |
| Sri Lanka (Ceylon) | An important source of good-quality sapphires, from very dark blue to very pale, some strongly particoloured. Also pink, yellow and pink-orange ‘padparadscha’. Crystal inclusions: particularly mica, pyrite, zircon crystals with haloes; elongate negative crystals; healed fractures often resembling fingerprints; rutile ‘silk’, two-phase inclusions. |
| Kampuchea (Cambodia) | Medium to good colour, in some cases similar to Thai stones. Small red crystals of uranium pyrochlore, plagioclase feldspar, crystals surrounded by healing fractures. |
| USA (Montana) | Untreated blue stones are usually very bright in appearance with an almost metallic lustre, frequently violet in tungsten light. Small well-formed crystals such as garnet, rutile, calcite and pyrite. Hexagonal zoning. Also pale green, yellow and pink stones. |
| Australia | Good to dark blue, dark greenish-blue and almost black sapphires. Yellow, green and parti-coloured stones also common. Strong zoning, feldspar, zircon crystals with associated haloes. |
Sapphires are also found in many other localities including Chine, Nigeria, Kenya, Tanzania, South Africa, Mozambique and Madagascar.
Tests
Dichroscope- Two different shades of blue. Although, some blue sapphires show blue plus green. Dichroic.
Spectrum—The absorption spectrum of blue sapphire often shows a single band in the blue. In some natural stones, absorption consists of a group of two or three close bands in the blue; a similar spectrum is seen in may green and golden stones.
Sapphire is the birthstone for September. In the middle ages it was said of sapphire that it preserved chastity, discovered fraud and treachery, protected from poison, plague, fever and skin dieases.
Quality Factors
Three important quality factors that affect value are colour, clarity and quality of cut. The highest value stones have a good rich colour, they are relatively free from inclusions and have a bright, lively appearance.