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If the light side of the slurriestone is a ToS the dark side might be Water of Ayre which should be finer. I don't know if they occur naturally joined but they used to be glued together a lot....

I agree with Disburden that the stone looks like a Thuringian. The slurrystone looks natural, the light side actually looks like a Tam O'Shanter I have, but I could be wrong. A close-up would help. A...