black tourmaline stone(semi-precious stone) The shape is good, high quality, worth having Natural Stones, non synthetic Typical chemical analysis:elementPercent by weight (%)SiO237.05Al2O329.8MgO8.05CaO1.18K2O0.09FeO8.33NaO21.19Cr2O30.22B2O310.16TiO20.44Typical 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 physical properties:Density2.82-3. ...