Sinhalite gemstone

by Pia Gujare

image of sinhalite gemstoneA gemstone discovered in 1952, formerly thought to be “brown peridots”. It may have confused with brown chrysoberyl, tourmaline or brown zircon in appearance.

Chemical composition: Borate of magnesium, iron and aluminium. The iron content can vary considerably and is the cause of the colour.

Crystal characteristics: Orthorhombic system with structure similar to peridot.

Habit: Fairly large pieces have been found as water worn pebbles. Complete crystals so far examined indicate a habit similar to that of peridot. The stone is much rare than peridot.

Varieties: From pale yellow or straw colour to dark brown and black depending on iron content. Colour can vary with the direction of cutting owing to dichroism. Some greenish-brown stones also occur.

Physical properties:

Cleavage: Cleavage cracks occur but their crystallographic direction is not yet known.

Hardness: 6.5

S.G.: 3.47 to 3.49

Lustre: Vitreous

Refraction: Double, Biaxial, negative sign; R.I.: 1.670 to 1.708; D.R.: 0.038

Spectrum: 4520, 4630, 4750, 4930 A° (Bands)

Dichroism: Distinct, colours green and brown, D.R. can be seen with a lens as a doubling of back facets.

Cut: Mixed and trap or step cut.

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