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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
--------Subgroup: Schorl Color: Black
--------Subgroup: Elbaite Types/Colors:
Achroite - Colorless or nearly colorless.
Verdelite - Green.
Indicolite - Blue.
Rubellite - Red to pink and peachy-pink.

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.

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