How do opal colours form?
Opal is composed of hydrated silica (SiO2.nH2O) and is made up of colloidal silica spheres.
To form precious opal, these silica spheres have to be arranged in ordered and closely packed arrays to diffract white light into different spectral colours. Silica spheres in these ordered arrays typically range in size from approximately 150 - 440 nm and their different sizes diffract different wavelengths of light. For example, spheres approximately 200 nm in diameter return blue light to the eye, while those which are 250 nm return green light, and those 320 nm return red light (see figure below).
In addition, the orderly arrangement of silica spheres has also resulted in the formation of distinctive parallel clusters of photonic colloidal crystals of precious opal, forming bands or patches of colour. Together, the ordered silica spheres combined with dislocations and twinning in these colloidal crystals, facilitates irregularly shaped regions of diffracted light. This leads to discreet patches of magnificent color when an Australian gem opal is rotated – also known as ‘play of colour’!
In contrast, potch opal (opal without colour) is made of a jumbled mass of silica spheres which does not diffract white light.
For further information on Australian opal formation please see Peacover 2019 and references within: Pecover, S. R. (2019). Frozen Opal Fluids and Colloidal Crystal Fire: Gem Opal Deposits in the Heart of Australia. InColor. Issue 41. Pages 34-59.