You really won't know until you tear into it. What you find will depend on the type of fireplace and the venting they used.

I'm reasonable certain it's masonry. An insert was added in time and a flu liner. The inside wall for some twelve feet is faux stone veneer. As stated earlier, I would expect to find sme metal lath behind the veneer to the left and right of the fireplace. The outside width is a third of that inside. Block with Faux veneer. I wonder what chance there may e that the inside veneer is attached to metal lath even across the fireplace?