Concretions are hard, compact masses of sedimentary rock formed by the precipitation of mineral cement within the spaces between sediment grains. They often exhibit spherical or discoidal shapes and can be found by weathering out of softer host rocks like shales or limestones. Collectors prize them for their unique internal patterns, especially when they form septarian cracks filled with secondary minerals.
Is this concretion?
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 a known reference. Concretion sits at Mohs 3-7 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Concretion leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Concretion typically shows a dull luster.
- 4Match the color rangeCompare against the expected color range: brown, gray, tan, black, yellow.
- 5Look at form & habitTypical habit: nodular.
Often confused with
Concretion vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside concretion
Minerals reported to co-occur with concretion. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Mohs hardness
- 3-7
- Density
- 2.0-3.0 g/cm³
- Streak
- White
- Luster
- Dull
- Transparency
- Opaque
- Crystal habit
- Nodular
- Cleavage
- None
- Rarity
- Common
- Uses
- Collector, Decorative
- Host rock
- Sedimentary Rock, Particularly Shale or Mudstone
- Typical price
- $5-50 for small specimens, up to $500 for large display pieces
Where rockhounds find concretion
1 mapped spotsClassic worldwide localities
- United States
- United Kingdom
- Canada
- Germany
- Morocco
Field-hunting tip
Look in sedimentary rock, particularly shale or mudstone country — that is the host setting where concretion typically forms. If you start seeing calcite, siderite, pyrite in float, you are in the right ground. Field specimens usually show a nodular habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Kansas — start trip planning there.
Common questions
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Find concretion on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
