Euclase is a beryllium aluminium hydroxide silicate mineral (BeAlSiO<sub>4</sub>(OH)). It crystallizes in the monoclinic crystal system and is typically massive to fibrous as well as in slender prismatic crystals. It is related to beryl (Be<sub>3</sub>Al<sub>2</sub>Si<sub>6</sub>O<sub>18</sub>) and other beryllium minerals. It is a product of the decomposition of beryl in pegmatites.
It was first reported in 1792 from the Orenburg district in the southern Urals, Russia, where it is found with topaz and chrysoberyl in the gold-bearing gravels of the Sanarka (nowadays probably, Sakmara River, Mednogorsk district, Orenburgskaya Oblast'). Its type locality is Ouro Prêto, Minas Gerais, Southeast Region, Brazil,