What is the best way to attach travertine stone to brick around the opening of a fireplace?
What is around your fireplace opening right now?
I would suggest you rig up a few tiles and run your fireplace. Many Travertine tiles are filled with a resin or epoxy that can change colour from the heat. I would test the effects of the heat on your tile.
Regular thin-set or dryset setting materials offer up very little flexibility so I would suggest you set the tile with a S2 thinset like Mapei's Grani-Rapid.