Thuringian is slate, thus more analogous to Welsh or Vermont slates than Arkansas stones.
Pulled from:Quote:
Thuringian waterwhetstones are slate stones or more precisely mud slate. These slates
consist of quartz – which is the abrasive material, clay, mica (glimmer) and chlorite.
The soft waterwhetstones were found in upper-Devonian age deposits.
http://strazors.com/uploads/images/articles/escher.pdf