Tonstein, German for "clay stone", is the general European term for bands of shale, mudstone, siltstone, and other clay-based sedimentary rocks. Occasionally it may refer to a specific rock sample from a tonstein.

Tonsteins are generally viewed as being the result of older (and generally minimal) volcanic activity. Once metamorphic and igneous activity has ceased, chemical and erosional activity have taken over, forming tonsteins and their more marine-based cousins, bentonites. Due to the long time of volcanic inactivity required for a tonstein to form, they are generally rare throughout the world.

Tonsteins are usually white or light-colored in nature, though they are often multi-colored due to the granular nature of its creation. Their primary components are the white multi-purpose mineral kaolinite (Al2Si2O5(OH)4) and quartz. They are primarily used as an indicator of nearby coal strata (also a product of volcanic activity), but are mined for the aforementioned kaolinite, as well as aluminum and slate.

Major tonstein belts are regularly mined throughout central Europe, Brazil, Canada, and parts of the coal belt in the United States.

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