Precious opal is defined by its characteristic play-of-color caused by the diffraction of light through a regular array of internal silica spheres. It is typically found in sedimentary ironstone nodules or as fillings in volcanic vesicles, prized by collectors for its vibrant spectral flashes.
Is this precious opal?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch precious opal with a known reference. Precious Opal sits at Mohs 5.5-6.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Precious Opal leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Precious Opal typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: white, black, blue, green, red, yellow, orange, multicolor.
- 5Look at form & habitCrystal system: amorphous. Typical habit: reniform.
Often confused with
Precious Opal vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside precious opal
Minerals reported to co-occur with precious opal. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- SiO₂·nH₂O
- Mohs hardness
- 5.5-6.5
- Density
- 2.0-2.2 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Amorphous
- Crystal habit
- Reniform
- Cleavage
- None
- Fluorescence
- Often Bright White, Yellow, Or Green Under UV
- Rarity
- Rare
- Uses
- Gemstone, Collector, Jewelry
- Host rock
- Sedimentary Rocks and Volcanic Cavities
- Typical price
- $50-500 per carat for high-quality specimens
Where rockhounds find precious opal
1 mapped spotsClassic worldwide localities
- Lightning Ridge, Australia
- Coober Pedy, Australia
- Welo, Ethiopia
- Querétaro, Mexico
- Dubník, Slovakia
Field-hunting tip
Look in sedimentary rocks and volcanic cavities country — that is the host setting where precious opal typically forms. If you start seeing quartz, chalcedony, goethite in float, you are in the right ground. Field specimens usually show a reniform habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Nevada — start trip planning there.
Common questions
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Find precious opal on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
