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Topaz (aluminum fluor-silicate)
Color: Blue, colorless, pink, orange, red, champagne, sherry-brown, brown and yellow.
Moh’s hardness: 8
Refractive index: 1.61-1.638
Critical angle: 38°
Specific gravity: 3.53-3.56
Cleavage: Very perfect in one direction.
Fracture: Conchoidal, uneven.
Dispersion: Medium (.014)
Heat sensitivity: None
Pleochroism: Dichroic – Blue: weak (light-blue to pinkish or colorless); Brown: definite (stronger yellowish cast); Green: definite (Pale-green to paler or colorless); Pink: definite (pink to pale pink or colorless); Red: strong (deep red to pinkish red); Yellow: definite (lighter to deeper yellow).
Birefringence (double refraction): Weak (.008-.01)
Crystal structure: Orthorhombic
Treatments: Blues are irradiated and heated.
Cutting: Orient the stone 7 to 10 degrees away from the cleavage plane.
Polishing: Alumina on most common polishing laps or 50,000 diamond on tin/lead or ceramic lap.
Comments: The cleavage on topaz is so perfect, that it looks almost like a polished surface. Some commercial cutters use the actual cleavage surface as the table. Therefore, before re-cutting a commercially cut stone, test the table on a polishing lap to ensure that it is not the cleavage plane.