Concretions are compact, spherical or irregular masses of sedimentary rock formed by the precipitation of mineral cement around a central nucleus, often a fossil. Collectors prize them because cracking them open can reveal perfectly preserved, three-dimensional fossils like shrimp, leaves, or ammonites inside. They are typically found in soft shale or mudstone formations where weathering helps expose the harder nodules.
Is this concretions with fossils?
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 concretions with fossils with a known reference. Concretions With Fossils sits at Mohs 3-5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Concretions With Fossils leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Concretions With Fossils typically shows a dull luster.
- 4Match the color rangeCompare against the expected color range: brown, gray, tan, black.
- 5Look at form & habitTypical habit: spherical to irregular nodules.
Often confused with
Concretions With Fossils vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside concretions with fossils
Minerals reported to co-occur with concretions with fossils. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Mohs hardness
- 3-5
- Density
- 2.5-2.7 g/cm³
- Streak
- White
- Luster
- Dull
- Transparency
- Opaque
- Crystal habit
- Spherical to Irregular Nodules
- Cleavage
- None
- Rarity
- Common
- Uses
- Collector, Decorative, Lapidary
- Host rock
- Shale and Mudstone
- Typical price
- $5-50 for small specimens, $100+ for large or museum-quality fossils
Where rockhounds find concretions with fossils
1 mapped spotsClassic worldwide localities
- Mazon Creek, Illinois
- Jurassic Coast, UK
- Western interior seaway, USA
- Alberta, Canada
Field-hunting tip
Look in shale and mudstone country — that is the host setting where concretions with fossils typically forms. If you start seeing calcite, pyrite, siderite in float, you are in the right ground. Field specimens usually show a spherical to irregular nodules habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Illinois — start trip planning there.
Common questions
How do you identify concretions with fossils?+
Where is concretions with fossils found?+
Can I find concretions with fossils in the United States?+
How much is concretions with fossils worth?+
What rocks look like concretions with fossils?+
What minerals are found with concretions with fossils?+
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Find concretions with fossils on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
