A mineral, Be4(Si2O7)(OH)2; density 2.59–2.6 g/cm3; hardness 6–7; orthorhombic: perfect cleavage; colourless or pale yellow; white streak; vitreous lustre; crystals thin, tabular, prismatic to needle-like; named after the French mineralogist Emile Bertrand (1844–1909), found in beryllium-bearing pegmatites in France and Russia, and may result from the alteration of beryl.