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The saw lines go from one layer to the other.

If you were going to set it in the box with plaster why would you cut it before?

Fill the hole with plaster and set the stone.

Or if it was an...

I will have to go with your observations of 2 layers as you are holding the stone not myself.

I have seen when you pull an old oil stone out of a wooden box it seems like there are 2 different...

That makes me think it could be from burnishing and the use of oil on one side and not the other.

How are you measuring the stone equation?

How are you determining that one side is harder than the other?

Oiled and burnished on one side would make it cut different.