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These are are real hits in school and at home for the little kids (and the big ones like me).
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A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil.Concretions are often ovoid or spherical in shape, although irregular shapes also occur.

The remains or impression of a prehistoric organism preserved in petrified form or as a mold or cast in rock.

A solid inorganic substance of natural occurrence.

An ornamental stone consisting of a hard variety of chalcedony, typically banded in appearance.