Composition: ZrSiO4
crystal System:  Tetragonal
Habit: Prismatic, square prisms terminated by pyramids, when the prism section is very short these resemble Octahedra.
Varieties. Transparent yellow, green, brown, red, orange, blue, golden brown, colourless.(reddish brown Zircon's from Indo-China are heat treated to produce blue, golden brown and colourless stones) The internal crystalline structure can be partially broken down by the presence of naturally occurring alpha particle irradiation, due to Uranium ar Thorium, for this reason Zircon can be defined as High, Intermediate or Low (Metamict) Zircons. Low Zircon are virtually non-crystalline and usually brown or green.
Refractive Index: 1.93-1.99 High Zircon. 1.8-1.93 Intermediate Zircon, 
1.78-1.84 Low Zircon
Double Refraction and Optic Sign: 0.059, positive, high Zircon, less for intermediate Ziron, and virtually zero for Low Zircon.
Dispersion: High 0.039
Specific Gravity: 3.9-4.68 (Low to High)
Hardness:  6.0-7.25 (Low to High) Zircon is brittle which causes "paper wear" damage when loose Zircon's are placed together is a stone paper and allowed to abrade each other.
Cleavage: Poor.
Lustre:   Vitreous to sub-adamantine.
Pleochroism: Weak except in heat treatewd blue stones
Absorption Spectrum: Diagnostic line at 653.5nm, often with other lines right across the spectrum
Occurrence:  Australia, Cambodia, France (Red), Indo-China, Sri Lanka, Thailand.


John Cadby Jewellers
 Goldsmith, Silversmith,
Diamond Merchants, Appraiser & Engraver.
3, White Hart Yard, Trowbridge, Wiltshire.  BA14 8BY
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