🍉 Tourmaline Optics Lab: Refractive Index 1.624-1.644, Dispersion and Brilliance of a Cut Tourmaline

Tourmaline ((Na,Ca)(Li,Mg,Al)3(Al,Fe,Mn)6(BO3)3Si6O18(OH)4 (elbaite)): refractive index 1.624-1.644, Mohs hardness 7-7.5, dispersion 0.017. Tourmaline is a family of complex boron silicates that come in a wider range of colours than any other gem, from pink rubellite and green verdelite to the neon blue-green of copper-bearing Paraiba stones. It has Mohs hardness 7 to 7.5 and a refractive index of about 1.62 to 1.64, with strong pleochroism, so the cut is oriented to show the best colour.