I think I'd like to get into the stone mantle business because I get a kick out of designing fireplace mantels. Of course there is a distinct advantage to stone (or in this case fake stone) over wood. Stone (or in this case concrete) is fireproof. Wood lends itself to more ornamentation and customization. But it will char if clearances aren't observed. Poured concrete will not. These designs were made with poured concrete in mind.  The first is horizontal over the firebox.  The second is virtually the same design but is arched above the firebox with a "thumbnail" molding around the firebox.