I saved a few pieces of granite tile from a bathroom I did and they were not very flat. Probably ok for putting edges on blades, but definitely not flat enough for lapping plane bottoms, IMO. Some tiles might be ok, but not the ones I had.
JP
If you have 3 of said plates, get some loose silicon carbide abrasive powder and lap them together and they will be flatter than any woodworkers needs anything.
Mix the abrasive with light oil...I use cheap olive oil or vegetable oil...until you have a thin paste. Label the plates A, B and C or 1, 2 and 3 or whatever it takes.
Smear a coating of paste on plate A, place plate B on top of it and move around in a figure 8. The pattern shouldn't be big...no more than 1/4 of the plate size and smaller than that is better.
Turn plate B 1/4 turn and continue until both plates have been cleaned up some...about half the surface...you should be able to see where it's cutting and where it hasn't. Oh...be sure to keep the plates wet with the paste.
Clean off the plates, saving as much of the paste as possible.
Now put plate B down, cover with paste and lap plate C on it, using the same pattern.
Clean.
Now plate C goes down and plate A goes on top. Continue.
Yes, you are seeing a pattern develop here and it should continue until all the plates have a uniform surface on them.
Store 2 of the plates somewhere safe...if you ever need to touch up your working plate, it'll go LOTS faster having those two plate rather than starting over.
Mike