Maybe like a flat dark gray stone/tile would do it like this...

What a wonderful room! I would add a pop of color such as these red onyx tile. Or a light tile such as these lovely mosaics.

Your room looks great already. And the black brick goes great with your other black accents. Before you add a different stone or brick, maybe just remove your brass and glass doors or just paint the brass black. Then, get an interesting grate. Here are a few suggestions.

Love the idea of just getting a new front grate and getting rid of the brass. Much cheaper and quicker!