If it is a gas fire then you don't have soot and blowback issues to consider. I'd probably try the limestone in that case.
I suspect the warmth will make it smell rather of creosote.
Our new mantlepiece is a large piece of green oak beeswaxed to seal it that was set into the wall in front of the RSJ that really holds up the opened out hearth. It has shrunk a bit but looks very nice.