Tonsteins are a distinctive type of rock, composed largely of the mineral kaolin. Tonsteins occur in association with coals in the Carboniferous strata of Europe.
The individual beds of tonstein are thin but laterally very extensive. The regional persistence of tonsteins suggests that they formed as a result of the alteration of volcanic ash falls.[1]
Tonsteins are tough indurated rocks, while younger kaolin claystones are generally softer, white, and plastic.[2]