Sinhalite

Until 1952, sinhalite was thought to be a brown variety of peridot, but on closer investigation it was found to be a completely new mineral. It varies from a pale yellowish brown to a dark greenish brown. Crystals have distinct pleochroism, showing pale brown, greenish brown, and dark brown when viewed from different directions. Although uncommon, sinhalite has been faceted for the collector. Cut stones may be confused with peridot, chrysoberyl and zircon.

Most gem quality sinhalite is found as rolled pebbles in the gem gravels of Sri Lanka. Crystals occur in Burma but are rare. Sinhalite is also found within the former USSR and non-gem quality material in the United States.

Sinhalite is named after "sinhala", which is the Sanskrit name for Sri Lanka.

Sinhalite Properties

Chemical Composition: 
Magnesium Aluminum Borate - MgAlBO4
Colors / Varieties: 
Yellow, dark green, greenish brown
Crystal System / Forms: 
Orthorhombic System / Grains and rolled pebbles.
Hardness: 
6.5
Cleavage / Fracture: 
Indistinct / Conchoidal fracture
Optic Character: 
Anisotropic, D.R.; Biaxial negative
Lustre: 
Vitreous
Refractive Index / Birefringence: 
1.670 - 1.708 / 0.038. Range: 1.667 - 1.711
Pleochroism: 
light brown, dark brown and greenish brown
Dispersion: 
0.018
Magnification: 
Crystalline inclusions, doubling
U.V. Fluorescence: 
Inert
Spectrum: 
Bands at 452nm, 463nm, 475nm, 493nm.
Cause of Color: 
Iron
Specific Tests: 
Doubling is seen.
Simulants (with separation tests): 
Peridot (pleochroism, spectrum), Tourmaline (optic figure, R.I. birefringence, S.G., spectrum), Chrysoberyl (R.I., birefringence, S.G., spectrum), Kornerupine (birefringence, spectrum), Epidote (R.I., birefringence, spectrum)
Geological Occurrence: 
In alluvial deposits, in limestones at granite contact zones.
Sources: 
Sri Lanka, Myanmar
Cuts & Uses: 
Facetted cuts, cabochon, etc.