I want to put up a stone veneer surround, around a gas fireplace we're going to install. I've seen everything from using 1/2" plywood and a metal lath, to putting the tiles directly on drywall. I have everything apart so I want to do this the best way.

The tiles/stones I will be using are quite heavy as they are real stone and not "manufactured stone". 

Items in play:
 - Weight - heavier tiles
 - heat - over and around a fireplace
 - Indoor