Summary: Most of the pale yellow to deep brown gemstones previously supposed to be iron-rich olivine² are a new mineral, sinhalite, having the ideal formula MgAlBO₄. Of 22 brown and yellow stones examined 20 are sinhalite and the remaining two are close to common green olivine. Sinhalite is readily distinguishable from green olivine by its higher refractive indices and specific gravity and its much smaller optic axial angle.