Zircon forms in the tetragonal system. It ranges from colourless to yellow, orange, brown, pink, red, green and blue, with many subtle shades in between. These colours are caused by traces of rare, radioactive elements in varying stages of radioactive breakdown. Colourless and blue zircons seen in jewellery have usually been artificially heat treated, since their occurrence in nature is rare. Because of its high lustre and dispersion ('fire') colourless zircon was used as a diamond substitute until it was displaced by synthetics such as cubic zirconia.
- Chemistry: Zirconium silicate
- Hardness: 7 - 7.5
- Refractive Indices: 1.78 - 1.99
- Specific gravity: 3.90 - 4.70
- Dispersion: High (0.039)
Zircons occur, sometimes with sapphire, in many of the alluvial deposits in eastern Australia that weathered from basaltic, volcanic rocks. Australia’s largest and most attractive gem zircons are found at Mud Tank, in the Northern Territory.
New South Wales
A number of sites in New South Wales, especially the sapphire fields of the New England area, produce attractive, large zircons.