Moh’s hardness: 7-7.5
Refractive index: 1.616-1.652
Critical angle: 38°
Specific gravity: 2.9-3.26
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.