Rocksday

Concretion

A hard, rounded lump formed when minerals cemented sediment around a center.

Concretion specimen
Photo: James St. John, CC BY-SA 4.0, via Wikimedia Commons

How to identify concretion

  • Round, oval or odd-shaped lumps that weather out of softer rock.
  • Harder than the surrounding layer.
  • Sometimes contain a fossil at the center.

Often confused with

Where it's found

  • Moeraki Boulders, New Zealand
  • Bowling Ball Beach, California
  • Utah (iron 'Moqui marbles')

Uses

Collector curiosities.

Rock tumbling

Can be tumbled with care.

Did you know?

Many 'dinosaur eggs' found by hikers are actually concretions.

Think you could spot it from a blurry close-up?

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