I was using a diamond plate.
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It looks like granite.
Dont underestimate how fine granite can be! treat it like an arky in regard to surface prep and I think you will keep being surprised. Nice score Stimpy!
I have wondered for a while how fine granite could be if it were flattened and lapped (but not polished), as long as the surface was non-porous.
I'm totally guessing that it's granite, though, given how many different types there are. I lived on a retired granite mine growing up, but it was mined for buildings. Very uniform, but it looked a LOT like that. The stuff that's used for chem lab tables would probably also be good.
I have a couple of slate stones that look almost exactly like that, but they are significantly harder to lap. They would have taken me hours by hand with a 325 DMT. They also both look like they are absorbing water, but when you let them soak the change in weight is insignificant.