A concretion with a fossil is a compact, spherical or irregular mass of sedimentary rock formed by mineral cement precipitating around a central nucleus, usually a fossil. Cracking one open can reveal a perfectly preserved, three-dimensional fossil such as a shrimp, leaf, or ammonite. It is typically found in soft shale or mudstone where weathering exposes the harder nodule.
Is this concretion with fossil?
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 concretion with fossil with a known reference. Concretion With Fossil 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. Concretion With Fossil leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Concretion With Fossil 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 found alongside concretion with fossil
Minerals reported to co-occur with concretion with fossil. 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
Where rockhounds find concretion with fossil
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 concretion with fossil 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 concretion with fossil photos
Common questions
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Find concretion with fossil on the map
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