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Large Blue Zircon Mounted in Centre of Cluster Ring
Large Blue Zircon Mounted in Centre of Cluster Ring

Introduction
Zircon is a naturally occurring gemstone, it should not be confused with the recently created and ubiquitous Cubic Zirconia. It is gemmologically interesting because it occurs in many varying forms. It is found in Sri Lanka, Mogok area of Burma, Espaly-Saint-Marcel in France, Arendal in Norway, Uralla, Sapphire Inverell in New England, New South Wales in Australia, and elsewhere.

General

Colour
Zircon occurs naturally in a large range of colours including: yellow, red, orange, brown, green, and blue. Heat treatment is often used to produce stones which are colourless, golden brown, or sky blue. Many of the available colours are very attractive, and zircons also have a high dispersion, and therefore show good "fire". Probably the best known colour is the very startling blue. Zircon is slightly brittle, and therefore should not be ill-treated, but otherwise it is quite durable and suitable for use in jewellery. Sadly it is not currently very popular, and this may be because its name is easily confused with the inexpensive synthetic cubic zirconia. (Cubic zirconia does not occur naturally).
Its great variability is caused by the natural radioactive decay of small amounts of uranium or thorium causing the crystal structure to break down into amorphous silica and zirconia. Heat treatment appears to reverse much of the deterioration in the crystal structure, so that "low" types can be converted successfully to "high" types. There are also intermediate types.

Technical Information
Chemical Composition and name:
Zirconium Silicate ZrSiO4;
usually there is partial replacement of the zirconium by iron, hafnium, thorium and uranium.
Hardness: 7 to 7.5 variable, even within a single specimen. "Low" types 6.5
Refractive Index: The RI of zircon varies considerably, the "high" type being 1.92 to 1.98; the bi-refringence 0.059 uniaxial and positive.


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