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The seller is a member here in the forum: Doorsch.
I am sure if you send a pm to him, he will answer all of your questions. Also I think he wouldn't sell the stone as thuringian if he was not sure...

+1 to what Double0757 said!
Go and try to find an original Escher or labelled small size thuringian or an unlabelled one from a trusted source. Most of them go for around 50 to 80$. They will do the...