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Corundum (aluminum oxide)

Types/Color:

• Synthetic Corundum – Synthetic is usually called corundum and is available in many colors.

• 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.

Moh’s 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

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.

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