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
2 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, 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.
More concretions with fossils photos
Common questions
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Find concretions with fossils on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
