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I think going though some sandpaper will give you some insight into how hard the stone is. Moreover, if used first it'll ease the DMTs burden.

That said...

I doubt highly that rubbing your DMT...

You know, I would buy some wet- dry sandpaper and place it wet on something flat (like my kitchen counter), and lap it that way were I unsure if it would kill my DMT. But that's just me...