Composition: Complex borosilicate of Aluminium, Magnesium and Iron
crystal System:  Trigonal
Habit: Three sided prism with convex faces heavily striated along prism length.
Varieties: Transparent to translucent single and parti-coloured (lenghtwise or radial) Akali rich stones contain sodium lithium or potassium and are colourless, red or green. Iron rich stones are dark blue, bluish green or black. Manganese stones are colourless or yellow brown to brownish black.
Refractive Index: 1.62-1.66
Double Refraction and Optic Sign: 0.014-0.021, negative
Dispersion: Low 0.018
Specific Gravity: 3.01-3.11 (black 3.15-3.26)
Hardness:  7.0-7.5
Fracture: Concohoidal
Lustre:  Vitreous
Pleochroism: Weak to strong depending on depth of body colour(two shades of body colour)
Absorption Spectrum: blue and green stones have a strong band in the green at 498nm, and a weak one in the blue at 468nm. Pink and red stones have a broad band in the green and two narrow bands in the blue at 450nm and 458nm.
Inclusions: Irregular thread-like cavities and flat films
Occurrence:  Brazil, East Africa, Namibia, Myanmar(Burma) The Malagasy Republic, Sri Lanka, USA & USSR.

John Cadby Jewellers
 Goldsmith, Silversmith,
Diamond Merchants, Appraiser & Engraver.
3, White Hart Yard, Trowbridge, Wiltshire.  BA14 8BY
Tel: 01225 776322  E-Mail: