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Corundum (aluminum oxide)
Types/Color
Padparadshah – Strong yellow, orange-pink to hot pink.
Ruby – Red to dark pink. Usually has a hint of violet.
Sapphire – Natural corundum is commonly thought of as dark blue, but it occurs in many other colors such as yellow, green, pink, colorless and purple.
Mohs hardness 9
Refractive index 1.766-1.774
Critical angle 34.5°
Specific gravity 3.99-4.0
Cleavage None
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
Cutting Corundum cuts and polishes at varying rates on different facets. Dont 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.