Deep South Opal · Tasmania
Australian Opal
What opal is made of
Opal is made from silica, a compound of silicon and oxygen, with water held within its structure. In precious opal, tiny silica particles form orderly arrangements. Too small to see with the naked eye, they help turn ordinary light into the colours that draw us to the stone.
Why opal flashes with colour
White light contains all the colours of the visible spectrum. In precious opal, regularly arranged silica particles scatter this light. Some light waves reinforce one another, making particular colours visible at particular angles. This effect is called diffraction. The flashes come from the stone’s structure, not from coloured pigment.
Turn the stone slowly. Blue may gather like dusk beneath the surface, then give way to green or a sudden flare of orange. A flash can disappear and return as the angle between the stone, the light and your eye changes. Nothing inside needs to move; the shifting view brings different colours into sight.
How opal forms in the earth
Many Australian deposits occur in weathered sandstone and mudstone. Groundwater carried dissolved silica through pores and cracks in the rock. As conditions changed, silica collected and gradually hardened, retaining some water. The details differ between deposits, but the result can be extraordinary: a quiet passage of water through the earth, eventually revealed in colour.
Australia’s opal fields
Each opal field has its own geology. Coober Pedy in South Australia is known for light opal; Lightning Ridge in New South Wales for black opal; and Queensland for boulder opal, often kept with its surrounding ironstone. “Crystal opal” refers to a stone’s transparency—how clearly light passes through it—not to a crystal-like atomic structure.
Explore further with Where Colour Lies
Our forthcoming illustrated book, Where Colour Lies — A Collector’s Guide to Australian Opal, takes this story further. It follows the science behind the colour, visits the opal fields and explains the practical side of collecting, from understanding a stone’s construction and treatments to caring for it at home.