|The Very Highest Quality Zircon Jewellery...|
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.
|Chemical Composition and Name||Zirconium Silicate ZrSiO4*|
|Hardness||7 to 7.5 variable, even within a single specimen. "Low" types 6.5|
|Refractive Index||Varies considerably, the "high" type being 1.92 to 1.98|
|Specific Gravity||4.60 to 4.70, usually 4.69|
* Usually there is partial replacement of the zirconium by iron, hafnium, thorium and uranium.
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