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It doesn't look like a Special Stone to me at all. There are other glittering hones though. Some Thuringians sparkle of pyrite. OP's pictures and description of his stone point in the direction of a British slate. At least one of those, a relatively lighter grey variety confusingly marketed as Yellow Lake, sparkles too. Not that the OP mentioned any sparkling, by the way.
Note 1: a few pictures and a concise description are often but a feeble basis for positive attributions.
Note 2: let's please stop calling anything and everything an Escher if it has no Escher label.