Sinhalite

Crystal system Orthorhombic
Transparency Transparent to translucent
Luster Vitreous
Fracture Conchoidal
Cleavage Indistinct
Specific Gravity 3.48
Hardness 6 to 7
Optical Character Biaxial- Double Refractive
Refractive index 1.668 - 1.707
Birefringence 0.036
Dispersion 0.017
Fluorescence
Pleochroism Strong; light brown, dark brown & green.
Chemical Formula MgAlBO4
Comments
Streak

Sinhalite (SIN-ah-lite) is one of the more recently discovered gemstones, since its true nature was revealed and it was adequately described in the early 1950's. It was long thought to be a brown variety of peridot; however, there were certain aspects of the stone that made some investigators suspicious.

Peridot in the range of properties in which sinhalite occurs has an intermediate index that is almost half-way between the high and low indices, giving an optic sign that is neither positive nor negative. The intermediate (or beta) index of sinhalite, on the other hand, is much closer to the higher figure, thus giving a very strongly negative sign. Dr. George Switzer, of the United States National Museum, took an x-ray powder photograph with a small amount scraped from the girdle of a large stone thought to be brown peridot and discovered that the pattern was not that of peridot. This fact was mentioned by the late Dr. William S. Foshag, then curator of Minerals at the Museum, to mineralogists at the British Museum of Natural History when referring to a brown stone on display as a peridot at the British institution. Although the usual practice in a situation of this nature would be for the original group to continue with the work and report the new mineral, the British undertook investigations quickly and published their findings. They called it sinhalite, after the Sanskrit name "Sinhala", for Sri Lanka, in which country the stone was found. It was shown to be a magnesium-aluminum borate, expressed by the formula MgAlBO4. Later, tiny crystals were identified in a primary deposit in a contact-metamorphosed limestone in Warren County, New York. Prior to this time, the only known specimens were those that had already been cut. Since it was first described, a number of both rough and cut sinhalites have been discovered in parcels of stones sent from Sri Lanka and in existing jewelry items.

Sinhalite, like peridot, crystallizes in the orthorhombic system, Gem material usually falls within a refractive index range of 1.665 - 1.671 for the low index to 1.704 - 1.710 for the high index. The birefringence varies from .036 to .039. The S.G. is from 3.47 to 3.50, its hardness is from 6 to 7, the luster is vitreous, the fracture is conchoidal, and the cleavage is indistinct. Its light-brown, greenish and darker brown pleochroism is strong, but the dispersion (.017) is weak.

Tourmaline, zircon and chrysoberyl can be separated from sinhalite by R.I, and/or S.G. The R.I, of sinhalite is perceptibly higher than that of peridot; moreover, it is clearly biaxial negative on the refractometer, whereas peridot is on the borderline. The absorption spectrum of sinhalite serves to assist in its separation from peridot: in addition to lines or bands in positions in the spectrum similar to those of peridot, it also has an extra band, which has been measured at 4630 A.U.