EUCLASE, a rare mineral, occasionally cut as a gem-stone. Like beryl, it is a silicate of beryllium and aluminium, but hydro gen is also present; the formula is or Be(A10H)Si0.,. It crystallizes in the monoclinic system as striated prisms terminated by acute pyramids. Cleavage is perfect, parallel to the clinopinacoid, and this renders the stone fragile with a tendency to chip, hence its name from the Greek e$, "easily," and ,cXctcts, "fracture." The colour is generally pale-blue or green, though sometimes the mineral is colourless. When cut it resembles cer tain kinds of beryl (aquamarine) and topaz, from which it may be distinguished by its specific gravity (3.1) . Its hardness (7.5) is rather less than that of topaz. Euclase occurs with topaz near Ouro Preto, Brazil, and with topaz and chrysoberyl in the gold bearing gravels of the river Sanarka in the southern Urals.