A trigonal mineral, 3(NaMg₃Al₆(OH,F)₄(BO₃)₃Si₆O₁₈) ; tourmaline group; forms series with schorl and with elbaite; in triangular or hexagonal prisms; pyroelectric and piezoelectric; in metamorphic and metasomatic lime-rich rocks; slices are used to measure transient blast pressures.