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
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