Claystone is a clastic fine-grained sedimentary rock composed of cemented clay-particles. With it grainsize it is comparable with mudstone and siltstone, but the particles need to be clay-size and a minimum of 50% of the rock needs to be composed of clay-sized particles.

Claystones often are mistaken for siltstone or mudstone, but the difference lies in the amount of clay-size particles present. To distinguish them the following definitions can be used:

• Siltstone – more than half of the composition is silt-sized particles.
• Claystone – more than half of the composition is clay-sized particles.
• Mudstone – hardened mud; a mix of silt and clay sized particles