Where is Boulder Opal found?
Hydrated amorphous silica (SiO2·nH2O) in ironstone matrix
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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 Cretaceous sedimentary sequence.
Collecting tips
- Look for Ironstone. Boulder Opal is always found within or attached to ironstone. Focus your search on areas with exposed ironstone formations, particularly those showing signs of weathering or fracturing.
- Identify Opal 'Potch'. Common opal (potch) often occurs alongside precious opal. While lacking play-of-color, its presence indicates favorable conditions for opal formation and can lead to precious opal discoveries.
- Examine Fractures and Voids. Opal typically forms in cracks, fissures, and voids within the ironstone. Carefully inspect these areas for any signs of silica infilling or color flashes.
- Observe Weathering Patterns. Weathering processes expose opal-bearing ironstone. Look for areas where the ironstone has been naturally eroded, revealing the opal within.
Quick Check
- Color:
- Vibrant play-of-color against a dark brown to reddish-brown matrix.
- Luster:
- Opal: Vitreous to waxy. Matrix: Dull to earthy.
- Streak:
- Opal: White. Ironstone matrix: Reddish-brown to brown.
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