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Fine scratch pattern and straight, chip free edge under magnification.

Compare it to a synthetic stone of known grit.

Put a kitchen knife to it and look at the stria?

That it was in a handmade box says it was prized by someone, but you have to remember, a 800, 1K edge is a fine thing for plane blade or chisel and...

Could be a Frictionite, not sure about the sizes, I know they made some large one and I have one about that size and another larger… somewhere.

Color is a bit off but, could be the photo or 50...