Unregistered

I'm tiling a slate floor for the first time and after looking at the tiles (ungraded, 'multi' slate) I am wondering - what do you do with the 'flaky' bits? Do you knock off the larger bits or will the sealer take care of it?
Also, my 'multi' slate does noy appear to be as, urm... 'multi' (ie lots of different burnt colours) as the shop demo board. Will the colours be highlighted after sealing or have I just got a duff batch? (the shop basically said 'natural product' when I asked them about it)
Steve.