Where is Boulder Opal found?
Opal in matrix (SiO2·nH2O)
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Open the appBoulder Opal occurs in specific geological environments. Knowing where to look, and in what kind of rock, is the difference between going home empty-handed and finding a great specimen.
Where to find Boulder Opal
Queensland, Australia
The primary source of Boulder Opal globally. Major fields include Koroit, Yowah, Quilpie, Eromanga, and Winton. These deposits are found within the Winton Formation, a Late Cretaceous sedimentary unit.
New South Wales, Australia
Some occurrences, though less significant than Queensland, can be found in areas like Lightning Ridge, where it is sometimes found in association with black opal.
Collecting tips
- Look for Ironstone Boulders. Boulder Opal is found within ironstone concretions or boulders. Prospectors look for these distinctive reddish-brown to dark grey rocks in weathered sedimentary terrains.
- Identify Opal Seams. Once a boulder is found, look for thin seams or veins of precious opal running through or coating the ironstone. These seams can be very thin, sometimes only a few millimeters thick.
- Observe Play-of-Color. The most definitive identification is the characteristic play-of-color (iridescence) exhibited by the opal when viewed under varying light conditions. This optical phenomenon is caused by the diffraction of light by ordered silica spheres.
- Check for Potch (Common Opal). Common opal (potch), which lacks play-of-color, often occurs alongside precious opal. Its presence can indicate the potential for precious opal nearby.
Quick Check
- Color:
- Vibrant play-of-color against a dark, earthy matrix.
- Luster:
- Vitreous to waxy on opal, dull to earthy on matrix.
- Streak:
- White for opal (if powdered), reddish-brown for ironstone matrix.
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