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Tourmaline
(boron aluminum silicates)
Color: Nearly every color including colorless and opaque
black. |
--------Subgroup: Dravite
Color: Brown, yellow-brown,
orange-brown and reddish-brown
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--------Subgroup: Schorl
Color: Black
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--------Subgroup: Elbaite
Types/Colors:
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Achroite - Colorless
or nearly colorless.
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Verdelite - Green. |
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Indicolite - Blue. |
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Rubellite - Red to pink
and peachy-pink. |
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Moh’s hardness: 7-7.5
Refractive index: 1.616-1.652
Critical angle: 38°
Specific gravity: 2.9-3.26
Cleavage: None
Fracture: Uneven, small conchoidal.
Dispersion: Medium (.017)
Heat sensitivity: Moderate
Pleochroism: Dichroic. Strong (many color
variations).
Birefringence (double refraction): Medium
(.014-.024)
Crystal structure: Hexagonal
Treatments: Heat treating to remove tan
color and silk. Irradiation to improve color.
Cutting: 1200 Nubond or 3000 regular lap may
create orange peel, which makes polishing difficult. If the “C” axis has a
bad color, or is closed (black or dark), it is best to cut the stone in a
rectangle or square shape. Orient the stone so the pavilion facets that are in
line with the bad “C” axis can be cut at a high angle such as 63-72 degrees.
This will reduce the interference of the undesirable “C” axis from being
reflected back through the crown.
Polishing: Alumina or cerium oxide on most
common polishing laps.
Comments: If possible, orient for best
color.
OPAL Server
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