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Topaz (aluminum fluor-silicate)

Color: Blue, colorless, pink, orange, red, champagne, sherry-brown, brown and yellow.

Names/Colors:

• Sky Blue - Light blue

• Swiss Blue - Medium blue

• London Blue - Dark blue, generally with steely undertone.

• Imperial - Orange, pink and red.

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.

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