Sinhalite

Sinhalite is a magnesium aluminum borate mineral (MgAl(BO₄)), long mistaken for brown peridot until it was recognized as a distinct species in 1952. Sinhalite forms in skarns, the contact metamorphic rocks that develop where boron-rich fluids from cooling granite intrusions meet magnesium-aluminum-bearing country rocks. The typical host is a dolomitic marble adjacent to a granite. As the intrusion sends hot fluids into the carbonate, new minerals crystallize in a narrow reaction zone, and sinhalite is among them when boron and aluminum are both available.

The Sri Lankan discovery story is a classic piece of gemological history. For decades, brownish stones from the Ratnapura alluvial gravels were traded as brown peridot. In 1952, gemologists reexamining parcels noticed refractive indices higher than peridot could produce. Careful analysis revealed a new mineral, and it was named after Sinhala, the traditional name for Sri Lanka. The source rocks themselves are long gone, weathered away and buried, but the gems survive in the river gravels that drained the ancient metamorphic terrain.

Mogok in Myanmar produces the second most important sinhalite, from marble-hosted deposits that still exist in place. The stones show strong yellow to green brown pleochroism, a direct consequence of the mineral's orthorhombic symmetry and its iron chromophore. Because sinhalite needs a specific chemistry of boron, magnesium, and aluminum in close contact, it remains genuinely rare, with only a handful of productive sources worldwide.

Sinhalite is an orthorhombic borate with refractive indices between 1.67 and 1.71, specific gravity of 3.47 to 3.50, and a vitreous luster. Color runs from yellowish brown through olive to dark brown, with the occasional greenish brown stone. The streak is white and the hardness 6.5 on the Mohs scale, suitable for most jewelry but softer than sapphire or chrysoberyl.

The most diagnostic feature is strong trichroic pleochroism. When rotated under a dichroscope, a sinhalite shows three distinct colors, typically pale brown, greenish brown, and deep yellowish brown. This separates it from peridot, which shows much weaker pleochroism and a simpler color shift. Birefringence sits near 0.038, higher than peridot.

Under magnification, sinhalite often contains two-phase inclusions and occasional healed fractures. Crystals in matrix are short prismatic and uncommon. Most gem material reaches the market as rolled pebbles from the Sri Lankan gem gravels, with flat, abraded surfaces typical of alluvial transport. The higher specific gravity is immediately noticeable when heft-testing against peridot of similar size.