Beryllonite is a rare sodium phosphate mineral with formula NaBePO4. The tabular to prismatic monoclinic crystals vary from colorless to white or pale yellowish, and are transparent with a vitreous luster. Twinning is common and occurs in several forms. It exhibits perfect cleavage in one direction. The hardness is 5.5 to 6 and the specific gravity are nα = 1.552, nβ = 1.558 and nγ = 1.561. A few crystals have been cut and faceted, but, as the refractive index is no higher than that of quartz, they do not make very brilliant gemstones
It occurs as a secondary beryllium mineral in granitic and alkalic pegmatites. It was first described from complex crystals and as broken fragments in the disintegrated material of a granitic vein at Stoneham where it is associated with feldspar , beryl. It was discovered by James Dwight Dana in 1888, and named beryllonite for its beryllium content.