Size (mm): 13.13 x 11.05mm
Weight (ct): 5.25ct
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 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.