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Corundum (aluminum oxide)
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
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.
Specimen Blue Sapphire 400 cts 35x35x30 MM 1889E Click here to see all rough Sapphire Click here to see...
Specimen Blue Sapphire 400 cts 35x35x35 MM 1890E Click here to see all rough Sapphire Click here to see...
Specimen Blue Sapphire 300 cts 40x30x25 MM 1717E Click here to see all rough Sapphire Click here to see...
Specimen Blue Sapphire Natural 190 cts 35x25x20 MM 186F The stone shown is the photo is the very stone...
Specimen Blue Sapphire Natural 200 cts 35x35x15 MM 187F The stone shown is the photo is the very stone...
Specimen Blue Sapphire Natural 140 cts 35x25x20 MM 188F The stone shown is the photo is the very stone...