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Zircon (zirconium silicate)
Color: Colorless, yellow, brown, red, blue, green, orange, violet and pink.
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Moh’s hardness: 6.5-7.5 --------Type: Normal (higher refractive index) Refractive index: 1.94-1.987 Critical angle: 31° --------Type Metamict (lower refractive index) Refractive index: 1.777-1.79 Critical angle: 34° Specific gravity: 3.9-4.71 Cleavage: Imperfect 2 directions. Fracture: Conchoidal, very brittle. Dispersion: Strong (.038) Heat sensitivity: None Moh’s hardness: 6.5-7.5 --------Type: Normal (higher refractive index) Refractive index: 1.94-1.987 Critical angle: 31° --------Type Metamict (lower refractive index) Refractive index: 1.777-1.79 Critical angle: 34° Specific gravity: 3.9-4.71 Cleavage: Imperfect 2 directions. Fracture: Conchoidal, very brittle. Dispersion: Strong (.038) Heat sensitivity: None |
 Blue Zircon (heat treated)  Cognac and Champagne Zircon  Yellow Zircon  Red Zircon  Bicolor Zircon |
Pleochroism: Dichroic –Blue: definite (blue to yellowish-gray or colorless) Brown: very weak (yellowish to reddish cast); Green: very weak (green to brownish-green); Red: weak (red to orange-red); Yellow: very weak (yellow to brownish-yellow).
Birefringence (double refraction): Strong (.06)
Crystal structure: Tetragonal
Treatments: Heat treating to improve color, usually from brown to blue.
Polishing: Alumina on tin or tin/lead; 50,000 diamond on most hard polishing laps. 50,000 or 100,000 on ceramic lap.
Comments: If possible, orient the stone to reduce the birefringence. Otherwise cut for best color and yield. Metamict (lower refractive index) zircon is usually found as river-worn pebbles, whereas normal (higher refractive index) zircon is usually found in crystal form.
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