Minerals and Metals in Ancient India v2.c43.s3.t6.25

v2.c43.s3.t6.25

Another gem mineral which has created some confusion is tourmaline. It is like garnet, a group of isomorphous substances with SiO₄ groups (also B₂O₅ groups) and possessing similar hardness (7-1/4) and same range of colour. The chemical formula has been tentatively suggested as H₉Al₃ (B.OH)₂ Si₄O₁₉ in which nine atoms of hydrogen are replaceable by variable amounts of Al, Mg, alkalies and colour-inducing Mn, Fe, Cr, etc. Mn induces the darker red shade, Cr³⁺ and Fe²⁺ green colour and so on. Tourmaline can be black, blue, green, red etc. Usually it is vertically striated and has resinous luster. Its specific gravity varies between 2.9 to 3.3. It is birefringent (two refractive indices) and dichroic (appearing differently coloured when viewed along different crystallographic axes).