Placing stone over metal fireplace face. Is this possible?
The manufacturer of the fireplace is okay with it, so you definitely can. I have done it many times and I found it the best option is to use cement board and then the appropriate application method for the finishing material. I've always used high temperature morter of various kinds and never had an issue. The key to remember is, accessibility to service the fireplace. Any openable panels must remain that way. Avoid man made stone that has epoxy in it as it tends to discolour. Many fireplaces I have installed are actually unfinished everywhere except the glass and require a stone finishing right up to it.

From the Safety data sheet