We had the same concerns on having a granite backsplash for the range area. We went with a 20" riser and then put granite above the riser up to the bottom of our vent hood.

Here is a picture:

I completely agree with steel behind a well-used range. The steel we have there (as opposed to the Sicis glass mosaic elsewhere) shows how much splashing gets on the back-I'd *hate* to be cleaning that off glass and grout. Much less off onyx! (I have onyx slices in my bathroom, well-sealed, but still I would really not want them behind my stove) The steel makes it *much* easier to cook with abandon.

The onyx will show elsewhere and people will completely understand and respect why there's steel on the 'business' section of the wall.

We used our glass tile on the wall behind the range. We figured, we can always start out with it, if it does happen to discolor badly over time, we can always get a taller riser for our range at a later date. We felt that a SS riser or splash behind the range would be likely to show some scorch marks, discoloration, and/or scratches, so it was a 6 of one, 1/2 dozen of the other type of decision for us.

FWIW, if I look closely, I can see the slightest am't of discoloration on some tiles right near the riser (only over one side of the range though). It seems as though it's more of a heat discoloration than cooking stains.

If that very faint discoloration is as bad as it ever gets, I'm fine with it. If it does get worse over time, then we'll consider the other option I mentioned.