Check you local building codes. There should be a list of dimensions required and acceptable materials
I used tile. Just make sure there is a bit of heat resistance in the material.
I agree with the answers above. If you'd like to see a tutorial on how to frame around a gas fireplace, you can check out our post in this link. We also show how we added cement board and stone ledge tiles around the insert to make it safe. I'm attaching a before and after picture so you can see how it turned out. Hope this helps!