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I think it is not plaster. Definitely too hard for plaster, And on both sides are similar "patterns" darker lines.

These uneven patches are a trace of oil, and a straight line is, in my opinion, a combination of layers

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You may be right that this oil influenced my feeling by the equation. It could also be misleading that on the edge of the stone you can clearly see two different layers which could suggest different...

I do not measure, but I feel that one side rubs easier and the other harder

The difference was perceptible during the stone equation.

I bought a stone at the flea market. Someone used it earlier with oil.
One side evidently harder than the other. In the first picture right after the purchase, then during the cleaning and equation....