Opal is a hydrated amorphous silica that is famous for its internal play-of-color in precious varieties. It typically forms in cavities or as a replacement mineral and is often found in sedimentary rocks or volcanic tuffs.
Is this 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 opal with a known reference. 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. Opal leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Opal typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: white, black, colorless, yellow, orange, red, green, blue, brown.
- 5Look at form & habitCrystal system: amorphous. Typical habit: botryoidal.
Often confused with
Opal vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside opal
Minerals reported to co-occur with 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
- 1.9-2.3 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Amorphous
- Crystal habit
- Botryoidal
- Cleavage
- None
- Fluorescence
- Often Fluorescent Green or Yellow Under UV
- Rarity
- Common
- Uses
- Gemstone, Collector, Decorative
- Host rock
- Sedimentary Layers, Volcanic Cavities, And Weathering Zones
- Typical price
- $10-100 for common opal specimens, $100-5000+ for high-quality play-of-color precious opal
Where rockhounds find opal
44 mapped spotsClassic worldwide localities
- Coober Pedy, Australia
- Lightning Ridge, Australia
- Queretaro, Mexico
- Welo, Ethiopia
- Virgin Valley, Nevada
U.S. states with opal
Each link opens a state-specific list of mapped rockhounding spots that produce opal.
Field-hunting tip
Look in sedimentary layers, volcanic cavities, and weathering zones country — that is the host setting where opal typically forms. If you start seeing quartz, calcite, goethite in float, you are in the right ground. Field specimens usually show a botryoidal habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Utah, Idaho, Nevada — start trip planning there.
Common questions
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Find opal on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
