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Well, I heard the ten commandments have been writen on tables of slate. Maybe the translation was not correct.....

Well, even a diamond consists of the same element ("C") as coal (mainly).

I really don’t understand, what’s so bad about slate. What is so negative knowing that every time you hone a razor on your yellow-green Escher, you are honing on a slate rock?
Maybe my translation...

Well,
sidmind is correct,
the Thuringian whetstones of the upper devonian age you know as water whetstones like the labelled Escher stones consist of slate. In Germany they are called...