These are spherical volcanic rock nodules that contain cavities filled with common or precious opal. When cracked open, they reveal a core of silica which may exhibit brilliant internal flashes of color or unique banded patterns.
Is this nodules filled with 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 nodules filled with opal with a known reference. Nodules Filled With 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. Nodules Filled With Opal leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Nodules Filled With Opal typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: white, gray, brown, multicolored.
- 5Look at form & habitCrystal system: amorphous. Typical habit: nodular.
Often confused with
Nodules Filled With Opal vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside nodules filled with opal
Minerals reported to co-occur with nodules filled 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
- 2.0-2.3 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Amorphous
- Crystal habit
- Nodular
- Cleavage
- None
- Fluorescence
- Often Fluorescent Green or White Under UV
- Rarity
- Common
- Uses
- Lapidary, Collector, Decorative
- Host rock
- Volcanic Rocks
- Typical price
- $10-200 depending on color play and size
Where rockhounds find nodules filled with opal
1 mapped spotsClassic worldwide localities
- Oregon, USA
- Queensland, Australia
- Mexico
- Brazil
Field-hunting tip
Look in volcanic rocks country — that is the host setting where nodules filled with opal typically forms. If you start seeing chalcedony, quartz, rhyolite in float, you are in the right ground. Field specimens usually show a nodular habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Oregon — start trip planning there.
Common questions
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Find nodules filled with opal on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
