So thuries are made of chalk? Ok. lol. Consider me schooled. ;) It must be very dense chalk I guess? I'm gonna go play tic tac toe with one. I'll report back. lol. Time to do some more research on this I suppose. I could have sworn I've read the words "thuringian slate" right here in the hallowed pages of SRP. But my dear friend, I must say, I don't believe you can judge the softness of the stone from a picture. :w Or the composition of it's mineral content :P
I will say that the dimensions of this stone do lend itself to tile setting. Now that I've thought on it a minute. lol. But I think it's a bit thick for that.
Ah I found it. Neil Miller says thuringian is a type of slate hone stone. So stifle that laugh! lol. I got the idea on good provenance. :)
Naturpark Thüringer Schiefergebirge - Obere Saale : Thuringian Slate Mountains - Franconian Forest
I really think I'm right about this...lol.