Kornerupine
Kornerupine is a rare boro-silicate mineral with the formula (Mg,Fe)4(Al,Fe)6(SiO4,BO4)5(O,OH)2. It crystallizes in the orthorhombic - dipyramidal crystal system as brown, green, yellow to colorless slender tourmaline like prisms or in massive fibrous forms. It has a Mohs hardness of 7 and a specific gravity of 3.3 to 3.34. Its indices of refraction are nα=1.660 - 1.671, nβ=1.673 - 1.683 and nγ=1.674 - 1.684.