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Corundum (aluminum oxide)
Moh’s hardness: 9
Refractive index: 1.766-1.774
Critical angle: 34.5°
Specific gravity: 3.99-4.0
Fracture: Uneven, conchoidal, splintery.
Dispersion: .Moderately high (018)
Heat sensitivity: None
Pleochroism: Dichroic. Darker colors usually show stronger dichroism. Natural blue: distinct (dark blue to greenish-blue); Natural green: weak (more or less yellow); Natural orange: strong (orange to yellow-brown or colorless); Natural purple: distinct (purple to pinkish); Natural ruby: strong (deep red to yellowish-red); Natural yellow: weak (yellow to light-yellow).
Birefringence (double refraction): Weak (.018)
Crystal structure: Hexagonal (trigonal), dipyramidal
Treatments: Heat treating to reduce silk is standard. Glass-filling voids in rubies is not considered an acceptable treatment.
Cutting: Corundum cuts and polishes at varying rates on different facets. Don’t cut with a bonded lap at any finer grit than 600, otherwise an orange peel effect will make polishing a nightmare. Pre-polish with 8,000 grit on ceramic, zinc or copper lap helps speed up final polishing immensely.
Polishing: Ceramic with 100,000 or 200,000 diamond spray, or zinc lap with 50,000 or 100,000 diamond. BATT can be used as a polish lap, but is a little soft for corundum.
Comments: Orient for best color and yield. Corundum cuts and polishes differently on different facets.