Spherulitic chert is characterized by radial clusters of microcrystalline quartz fibers known as spherulites embedded within a siliceous matrix. It is highly sought after by lapidary artists for the distinctive 'starburst' or 'orb' patterns visible when the material is cut and polished. Collectors typically find this material as nodules or beds within volcanic ash deposits or sedimentary strata.
Is this spherulitic chert?
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 spherulitic chert with a known reference. Spherulitic Chert sits at Mohs 6.5-7 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Spherulitic Chert leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Spherulitic Chert typically shows a waxy luster.
- 4Match the color rangeCompare against the expected color range: white, gray, tan, brown, black.
- 5Look at form & habitCrystal system: trigonal. Typical habit: spherulitic.
Often confused with
Spherulitic Chert vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside spherulitic chert
Minerals reported to co-occur with spherulitic chert. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- SiO₂
- Mohs hardness
- 6.5-7
- Density
- 2.6-2.65 g/cm³
- Streak
- White
- Luster
- Waxy
- Transparency
- Opaque
- Crystal system
- Trigonal
- Crystal habit
- Spherulitic
- Cleavage
- None
- Rarity
- Common
- Uses
- Lapidary, Collector
- Host rock
- Sedimentary or Volcanic Environments
- Typical price
- $5-30 for rough slabs
Where rockhounds find spherulitic chert
1 mapped spotsClassic worldwide localities
- Oregon
- Wyoming
- California
- Mexico
- Turkey
Field-hunting tip
Look in sedimentary or volcanic environments country — that is the host setting where spherulitic chert typically forms. If you start seeing quartz, chalcedony, jasper in float, you are in the right ground. Field specimens usually show a spherulitic habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in California — start trip planning there.
Common questions
How do you identify spherulitic chert?+
Where is spherulitic chert found?+
Can I find spherulitic chert in the United States?+
How much is spherulitic chert worth?+
What rocks look like spherulitic chert?+
What minerals are found with spherulitic chert?+
What kind of rock does spherulitic chert form in?+
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Find spherulitic chert on the map
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