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Zircon |
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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* |
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 |
Bi-refringence | 0.059 |
Optic Sign | Positive |
Optical Character | Uniaxial |
Specific Gravity | 4.60 to 4.70, usually 4.69 |
Notes
* Usually there is partial replacement of the zirconium by iron, hafnium, thorium and uranium.
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