I bought some sealed from Diespecker when I picked up the tiles, pretty pricey at 40 quid a tin but I remember having a black marble worktop in a previous flat which was HELL to keep safe/clean/undamaged so would recommend using something decent on real stone.
I bought a tin from the supplier too, which they said should be enough. I'm a bit wary of oil stains, but the popularity of terrazzo and the lack of disaster stories online suggests this may be unwarranted.
How have you sealed them? I've just spent a small fortune on terrazzo kitchen tiles.