What causes the colour in opal?

The play of colour in precious opal comes from the diffraction of light through its internal structure of tiny silica spheres arranged in a regular grid. When the spheres are uniform in size and evenly spaced, they split white light into spectral colours that shift as the viewing angle changes.

Opals rate 5.5 to 6.5 on the Mohs scale, making them softer than most other gemstones in jewellery, and they contain water in their structure so they can crack if they dry out suddenly. They suit earrings and pendants for everyday wear better than rings that take daily knocks, and they should be kept away from heat and dry conditions.

What is the difference between black opal and white opal?

The body tone of the opal is what varies: black opals have a dark body colour that makes the play of colour look vivid and dramatic, while white or light opals have a pale background that produces a softer, milkier shimmer. Black opals from Lightning Ridge in New South Wales are the most valuable Australian opals.

What is an opal?

By Jared James · Last updated 21 May 2026

Definition

Opal is a mineraloid, not a crystal, formed from silica and water, and it is prized for its play of colour, the shifting spectral flashes that move across the surface as the stone is tilted. Precious opals display this iridescence while common opals do not. Australia produces around 90% of the world's precious opal, including black opals from Lightning Ridge, white and crystal opals from Coober Pedy, and boulder opals from Queensland. Opal is the national gemstone of Australia and a birthstone for October.

Frequently asked questions

What causes the colour in opal?: The play of colour in precious opal comes from the diffraction of light through its internal structure of tiny silica spheres arranged in a regular grid. When the spheres are uniform in size and evenly spaced, they split white light into spectral colours that shift as the viewing angle changes.
Are opals fragile?: Opals rate 5.5 to 6.5 on the Mohs scale, making them softer than most other gemstones in jewellery, and they contain water in their structure so they can crack if they dry out suddenly. They suit earrings and pendants for everyday wear better than rings that take daily knocks, and they should be kept away from heat and dry conditions.
What is the difference between black opal and white opal?: The body tone of the opal is what varies: black opals have a dark body colour that makes the play of colour look vivid and dramatic, while white or light opals have a pale background that produces a softer, milkier shimmer. Black opals from Lightning Ridge in New South Wales are the most valuable Australian opals.

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