Salvaged ceramic stone?

The sharpening heathen returns, I know I probably belong in r/unsharpening but I'm moving up slowly.

I have an old stone tile (not entirely sure of the stone, but it's some fancy Italian thing from the PIL remodel) I use to flatten my dying oil/water stone. I have a curved piece of ceramic I found, and I've spent the last few days, on and off while I've nothing better to do, running it on the flattening tile. It's very-slowly gettin flatter, so it must be softer than the tile.

Any way to find out a rough grit level for this? It's not overly important, but I am curious. Also, if the tile is the one that grinds everything else should I finish my knifes on it (pre-strop).

(Reconditioned oilstone, degreased for water use and flattened- previously posted in its sorry old state, felt bad for it, but not posting about it again