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Here is a black stone that had an Escher, “Dark Blue” end label (blue).

Based on the slurry and the original photo probably a slate, maybe a Thüringen.

Hard black stones like Arks will slurry...

Nice, what color is the slurry?

Can you post some photos of the sides and end?

What are the dimensions?

Look like a nice stone, looks slate-ish from the photo.

Thanks for posting it.