Using concrete and a frame sounds like a really good way to construct a hearth. It's strong and is fairly easy to put together. After it's in place, all you have to do is use your imagination and put any nonflammable material on it or create an unusual design.
We lived in a house for a while that had a brick hearth. I never thought that it was especially attractive.
I can't imagine any one using a material that was flammable to make a hearth because sparks do fly out of the fireplace opening. Concrete is the best.