Where is Precious Opal found?
Opal (SiO2·nH2O)
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Open the appPrecious 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 Precious Opal
Australia
The world's primary source of precious opal, particularly from South Australia (Coober Pedy, Mintabie, Andamooka), New South Wales (Lightning Ridge - famous for black opal), and Queensland (boulder opal).
Ethiopia
Significant deposits of Welo opal (hydrophane opal) have emerged, known for their vibrant play-of-color and often found in volcanic rocks.
Mexico
Known for fire opal (with a fiery orange to red body color, sometimes with play-of-color) and water opal, often found in rhyolitic volcanic rocks.
Brazil
Produces some precious opal, often found in sedimentary formations.
USA
Limited deposits in Nevada (Virgin Valley - black opal, often unstable), Idaho, and Oregon.
Collecting tips
- Look for specific geological environments. Search in areas known for sedimentary basins with silica-rich solutions, weathered volcanic rocks (rhyolite, basalt), or hot spring deposits. In Australia, look for 'opal dirt' (weathered claystone or sandstone) and 'nobbies' (rounded opal concretions).
- Observe light interaction. Precious opal's defining feature is its play-of-color. Rotate suspected specimens under a strong light source to look for flashes of spectral colors. Common opal (potch) will not exhibit this phenomenon.
- Check for specific gravity and hardness. Opal is relatively light (SG 1.98-2.25) and soft (Mohs 5.5-6.5). It can be scratched by quartz. These properties can help differentiate it from harder, denser minerals.
- Examine fracture patterns. Opal exhibits a conchoidal (shell-like) fracture, which is characteristic of amorphous materials.
- Be aware of stability. Some opals, particularly those from volcanic environments (e.g., Virgin Valley, Nevada), can be unstable and prone to 'crazing' (cracking) due to dehydration. Hydrophane opals (e.g., Welo opal) can absorb water and change transparency and play-of-color.
Quick Check
- Color:
- Play-of-color (flashes of spectral colors); various body colors.
- Luster:
- Vitreous to waxy/resinous.
- Streak:
- White.
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