What is Opal?

 An opal is a 'gemstone' - that is, a mineral valued for its beauty. Gemstones are most often used in jewellery and examples include diamonds, rubies, emeralds, sapphires, jade, opals and amethysts. Gems generally get their colour because of certain metals contained in the mineral (for example purple amethyst is quartz containing tiny amounts of iron). However opals are unique because they display a rainbow-like display due to their intrinsic microstructure which diffracts white light into all the colours of the spectrum.

Opals are multi-coloured and consist of small spheres of silica arranged in a regular pattern, with water between the spheres. The spheres diffract white light, breaking it up into the colours of the spectrum. This process is called 'opalescence'. Larger spheres provide all colours, smaller ones only blues and greens. Opals that have a predominantly red colour are very rare as they only occur where where larger silica spheres were deposited.

White opals have delicate, pale colours on a lighter background. Black opals (very rare and valuable) have a dark background and colours ranging from brilliant red through to greens, blues and purples. Boulder opals are cut with the natural host rock, ironstone, on the back.

There is some uncertainty around the origin of the word opal - it may come from the Greek opallos meaning 'to see a change (of colour)' or may be Sanskrit for ‘(precious) stone’.

 

Courtesy Geoscience Australia: http://www.ga.gov.au/education/classroom-resources/minerals-energy/australian-mineral-facts/opal