Zircon

Zircon is an attractive gemstone that delivers good brilliance in faceted stones. It occurs in a range of natural colours and although colourless stones are rare, they may be produced by heat-treating brown stones. Colourless zircon was once a common diamond substitute, it has a high level of dispersion providing a great deal of fire. This was complimented by an exceptional lustre, giving the stone the appearance of a diamond.
Zircon is often confused with cubic zirconia (CZ), a synthetic material. Zircon is a zirconium silicate, whereas CZ is a zirconium dioxide.

Source locations for Zircon include: Australia, France, Myanmar, South Africa, Sri Lanka, Thailand and the USA.

Properties of the Gemstone and Additional Information

Gemstone type: Zirconium Silicate - ZrSiO4. Zircon occurs as High zircon and Low zircon.
Low zircon is termed metaminct as the crystal lattice has been damaged by radiation. It is rarely cut as a gemstone.
Crystal System: Tetragonal.
Chemical Composition: Zirconium Silicate, ZrSiO4
Colour: Colourless, red, brown, yellow, green, blue, violet.
Lustre: Bright vitreous to sub-adamantine (approaching that of diamond).
Pleochroism: Dichroic: weak for natural stones, strong for heat-treated stones (blue and colourless).
Dispersion: High
Hardness: 7 1/2 on the Mohs' scale for High zircon (Low zircon is 6 1/2).
Toughness: Poor: Although fairly hard, zircon is brittle, the facet edges may be damaged by knocks.
Cleavage: Indistinct
Density in gm/cc: 4.6 to 4.8 for High zircon (Low zircon 3.9 to 4.6).
Double refraction: Yes
Refractive index: Low 1.78 to1.84, High 1.92 to1.99.

Common Treatments

Zircon may be heat-treated to improve the colour. The resulting colour may fade with prolonged exposure to strong light.

Most sky blue stones are heat treated material.
Light brownish stones are heated to produce colourless stones.
Yellow-brown to reddish stones are heat treated to reduce the brown colour and may produce golden colours.