Kristin: I plan to do this too! Here is the way I plan to do it...
(then I will do dark wax that will settle into places.)
https://www.youtube.com/watch?v=ZBQFvb-5xVo It's starts at 2:18 with the fireplace!
Also You can use a joint compound and a chalk based or mineral based paint to achieve the same look!
Here is one idea I thought might help you Kristi, https://www.hometalk.com/27265581/installing-stone-veneer-on-outdoor-fireplace