Opal (hydrous silicon dioxide)
Color: Colorless, yellow, orange, red and brown.
Mohs hardness: 5-6.5
Refractive index: 1.44-1.46
Critical angle: 43.5°
Specific gravity: 1.9-2.23
Cleavage: None
Fracture: Conchoidal, brittle, splintery.
Dispersion: None
Heat sensitivity: High to moderate
Pleochroism: None
Birefringence (double refraction): None
Crystal structure: Amorphous
Polishing: Cerium on most common polishing laps.
Comments: Care should be taken when dopping to prevent damage to the stone by thermal shock. Stones that display opalescent fire are usually better to cut as cabochons.
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