... chemical formula:(Ca,K,Na,[])(Al,Fe,Li,Mg,Mn)3(Al,Cr, Fe,V)6(BO3)3(Si,Al,B)6O18(OH,F)4Typical ... granite)Crystal systemTrigonalCleavageIndistinctFractureUneven, small conchoidal, brittleLusterVitreous, sometimes resinousStreakWhiteSpecific gravity3.06 (+.20 -.06)Polish lusterVitreousOptical propertiesDouble refractive, uniaxial negativeRefractive indexnω=1.635–1 ...