Mounting a TV above a fireplace with an unknown stone
Mounting a TV above a fireplace is an overall bad thing to do.


Fireplaces get hot. Electronics do not like heat.
Above fireplace places TV at a less than optimum viewing angle. TVs are designed for best image looking mostly straight on at eye level whilst sitting on a typical couch.
Getting connection power cords and signal cables up around the side of the fireplace and to the area of the bracket is totally hampered by the firebox and chimney that is inside the wall. Normal in wall hiding of cables is not possible and leads to an ugly installation.