🌈 Opal Optics Lab: Refractive Index 1.37-1.47 (usually about 1.45), Dispersion and Brilliance of a Cut Opal

Opal (SiO2·nH2O (typically 3-10 wt% water)): refractive index 1.37-1.47 (usually about 1.45), Mohs hardness 5.5-6.5, dispersion 0.012. Opal is hydrated, non-crystalline silica (SiO2·nH2O) with a Mohs hardness of only 5.5 to 6.5 and a low refractive index of about 1.45. In precious opal the flashes of rainbow colour are not caused by pigment: they come from diffraction of light by a regular array of tiny silica spheres, which is why the colours change as the stone is turned.