Answer: tourmalines

AnswerCrossword Clue
tourmalinesA brittle gray or black mineral that occurs as prismatic crystals in granitic and other rocks. It consists of a boron aluminosilicate and has pyroelectric and polarizing properties, and is used in electrical and optical instruments and as a gemstone
tourmalinesTOURMALINE, (French) a brittle boron aluminosilicate mineral with a glassy lustre, occurring as prismatic crystals of the trigonal system