Great Question and I have a simple answer! It came to me quickly so you know it's good! Paint the mirror a shade lighter than the fireplace box. I'd suggest Pittsburgh Paints Dover Gray 1001-5. This will help unite the entire assembly and bring out some of the darker stones in the wall. Good luck!

The mirror should move to another location and be replaced with art. If this was a picture you found in a store would you purchase it to go over the mantel? It looks like a big white hole in the fireplace.

If not a painting then a wall sculpture in metal or wood.

I see white ceiling in the mirror. Is that what you want to see twice? Mirrors just double what they reflect, so re-think it. Art nearly as wide as the firebox would look much better. Otherwise, paint the mirror one of the darker colors from the FP stone.