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Zircon (zirconium silicate)

Color: Colorless, yellow, brown, red, blue, green, orange, violet and pink.

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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