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. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: waxy.
- 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
Where rockhounds find spherulitic chert
1 mapped spotsClassic worldwide localities
- Oregon
- Wyoming
- California
- Mexico
- Turkey
Field-hunting tip
Where it forms: sedimentary or volcanic environments. If you start seeing quartz, chalcedony, jasper in float, you are in the right ground. Typical habit: spherulitic. 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.
More spherulitic chert photos
Common questions
How do you identify spherulitic chert?+
Where is spherulitic chert found?+
Can I find spherulitic chert in the United States?+
What rocks look like spherulitic chert?+
What minerals are found with spherulitic chert?+
What kind of rock does spherulitic chert form in?+
What is spherulitic chert used for?+
Find spherulitic chert on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
