Are there any grout lines or any other tell tale signs of how it was formed? How about a close up pic.

Yes, here are some more pictures.

If it is within your budget, see if it would be possible to have all of it refaced with tile. Maybe a continuous slab for the hearth piece.

It might be MDF. Fiberboard and tried a faux finish. Maybe you can test it somewhere under the mantle where you can't see it. Take a sharp screw driver and hammer and try to see if it is hardwood or soft or if it sounds like a stone or cement.

What does it sound like when you knock on it?

The mantel looks like plaster to me. Cement would be difficult to cast in place and/or mount. The hearth appears to be ceramic tile. I don't mind the hearth, although given it's proximity to the f/p, if you want to change the colors, you have limitations because of what the materials seem to be. I agree with dclostboy, what does it sound like when you knock on it? I wish you well with your endeavor. Happy Houzzing.

Thanks, dc. I went through Houzz's "Help" and asked if there was a way for houzzers to page each other and they responded that they did not. I think I'll ask if they will add an optional blind contact system. It would probably cost a few million bucks, but hey! it won't hurt to ask. I appreciate your responding. :)))))

It's sounds as if I'm knocking on a rock. It's pretty darn solid.

@happyasaclam, It would be awesome if we could add audio in here too. Maybe you can add that to your request! :)

LOL, ajocson, I will! I was thinking about sound this morning! Great minds, right? :)))

Perhaps it is plaster over wood, then, given it's solid tenor when thumped. I wouldn't want to cause you grief in your process--I wonder if it makes sense to get a small masonry bit, and drill a small hole into the mantel, say 1/4" max near where the mantel and the wall meet. If it gets easier to drill after a short distance, then it is likely that wood lies underneath, If this is the case, you have some options for either removing the topping material and replacing it with tile or refinishing or painting the mantel below. If the drilling is resistant to penetration after 3/4" or so, then perhaps it is concrete, and then your options are basically to remove, paint or stain it. Confirm that you are using the correct finish for the surfaces in proximity to the f/p itself, before moving ahead, in either case. Happy Houzzing.

DMH, I agree with the way to check the material. I didn't know your procedure. Much better way to do it : ) Thanks