You would need some contractor input on that. I imagine it depends on whether you're able to apply some kind of base over the existing rock in order to create a flush surface for application of stone slab or tile on top. If that's the route you take, you will be moving forward the front face of the finished chase away from your firebox. Now would be the time if you have an existing firebox you are thinking of replacing. Then you could relocate the new unit slightly forward if you prefer everything to be flush like the photo. I suspect a contractor would advise on removing the existing rock rather than applying product over top, but it will all depend on your particular set-up.