Crystal opal is a transparent to semi-transparent variety of precious opal renowned for its brilliant internal play-of-color resulting from the diffraction of light by microscopic silica spheres. Unlike common opal, it exhibits a distinct spectral flash when rotated and is highly prized for its clarity and brightness. Collectors should examine the stone against a light source to verify transparency and look for sharp, multi-colored 'fire' patterns.
Is this crystal 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 crystal opal with a known reference. Crystal 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. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: white, black, colorless, blue, green, yellow, red, orange.
- 5Look at form & habitCrystal system: amorphous. Typical habit: nodular, botryoidal, reniform, massive.
Often confused with
Crystal Opal vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside crystal opal
Minerals reported to co-occur with crystal 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
- 1.9-2.2 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Amorphous
- Crystal habit
- Nodular, botryoidal, reniform, massive
- Cleavage
- None
- Fluorescence
- Often fluorescent white, green, or yellow under UV
- Rarity
- Rare
- Uses
- Gemstone, Jewelry, Collector
- Host rock
- sedimentary ironstone, volcanic rhyolite, or sandstone cavities
Where rockhounds find crystal opal
Classic worldwide localities
- Coober Pedy, Australia
- Lightning Ridge, Australia
- Welo, Ethiopia
- Queretaro, Mexico
- Virgin Valley, USA
Field-hunting tip
Where it forms: sedimentary ironstone, volcanic rhyolite, or sandstone cavities. If you start seeing quartz, montmorillonite, limonite in float, you are in the right ground. Typical habit: nodular, botryoidal, reniform, massive. Train your eye for that shape before scanning the outcrop.
More crystal opal photos
Common questions
How do you identify crystal opal?+
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What rocks look like crystal opal?+
What minerals are found with crystal opal?+
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Find crystal opal on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
