Where is Opal found?
Hydrated amorphous silicon dioxide (SiO2·nH2O)
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Open the appOpal 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 Opal
Australia
The world's primary source of precious opal, particularly black opal (Lightning Ridge, New South Wales), white opal (Coober Pedy, South Australia; Mintabie, South Australia; Andamooka, South Australia), and boulder opal (Queensland). These deposits are typically found in Cretaceous sedimentary basins.
Ethiopia
A significant source of Welo opal (hydrophane opal) and other precious opals, often found in volcanic rocks (rhyolites) in the Wollo Province. These deposits are relatively recent discoveries but have become major producers.
Mexico
Known for its vibrant fire opals, which are typically orange, red, or yellow, often without play-of-color, but some exhibit it. These opals are found in volcanic rocks, particularly rhyolites, in states like Querétaro, Jalisco, and Hidalgo.
Brazil
Produces common opal and some precious opal, often found in sedimentary formations.
United States
Various localities produce common opal and some precious opal, including Nevada (Virgin Valley black opal in volcanic ash), Idaho, Oregon, and California. Oregon is known for its common opal and some precious opal in volcanic settings.
Hungary (Slovakia)
Historically a significant source of precious opal, particularly from the Dubník mines, which were active for centuries. These deposits are associated with volcanic activity.
Collecting tips
- Look for Silica-Rich Environments. Focus on areas with evidence of past or present silica-rich groundwater activity, such as weathered volcanic rocks (rhyolite, basalt), sedimentary basins with sandstone or diatomaceous earth, or hot spring deposits.
- Examine Host Rocks. Opal often forms in veins, nodules, or as replacements within host rocks. Look for cracks, fissures, or cavities in sandstone, ironstone (boulder opal), or volcanic ash beds.
- Observe for Play-of-Color. When searching for precious opal, rotate potential specimens under natural light to detect the characteristic play-of-color. Even small flashes can indicate the presence of precious opal.
- Check for Botryoidal or Reniform Habits. Common opal often forms in rounded, grape-like (botryoidal) or kidney-shaped (reniform) masses, which can be indicators of opalization.
- Consider Hydration. Opal's water content can make it susceptible to 'crazing' (cracking) if it dries out too quickly. Handle specimens carefully and consider storing them in a humid environment if they appear to be hydrophane (water-absorbing) or freshly collected.
Quick Check
- Color:
- Highly variable, from colorless to white, black, gray, red, orange, yellow, green, blue, pink. Precious opal shows 'play-of-color'.
- Luster:
- Vitreous to waxy or resinous.
- Streak:
- White.
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