Zircon
Zircon is most famous for its colourless stones, which closely resemble diamonds and have been used both intentionally and mistakenly in their place. Although colourless when pure, impurities will produce yellow, orange, blue, red, brown, and green varieties. Brown stones from Thailand, Vietnam, Kampuchea are usually heat treated to change them into the colourless or blue stones popular in jewellery. Blue stones that revert to brown will regain the blue if reheated. Blue zircon reheated in the presence of oxygen will change to golden-yellow. Zircon may be distinguished from diamond by its double refraction and by wear and tear on its facet edges. It has been imitated by both colourless glass and synthetic spinel. Some zircon contains radioactive thorium and uranium, which eventually break down the crystal structure. Decayed stones are known as “low” zircon with a “metamict” structure, undamaged materials is “high” zircon.
Occurrence
Gem-quality crystals are usually found as pebbles in alluvial deposits. Sri Lanka has been a source of gem material for over 2,000 years; other localities include Burma, Thailand, Cambodia, Vietnam, Kampuchea, Australia, Brazil, Nigeria, Tanzania, and France.
Remark
Zircon was believed to provide the wearer with wisdom, honour, and riches, and loss of lustre was said to warn off danger. The name is from the Arabic zargun, which derives from the Persian for “gold colour”.
| Crystal structure | Tetragonal |
| Composition | Zirconium silicate |
| Hardness | 7.5 |
| SG | 4.69 |
| Rl | 1.93-1.98 |
| DR | 0.059 |
| Lustre | Resinous to adamantine |
