Claystone is a clastic sedimentary rock. It is composed of very fine particles (clay sized, less than 1/256 mm) which have become cemented into a hard rock.

Claystone is distinguished from a mudstone by the mudstone's softening upon exposure to water. A claystone is also distinct from a clay shale which has fine layers that easily split.

See Steve's video interview about asteroid capture at Moonandback:
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