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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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