Hi there
If you are in the US of A then you should check with your local inspectors who will advise you correctly of the regulations pertaining to fireplace insert pipework.
Down here where the sun shines first in the World we do use copper pipe for most if not all gasfitting work and sleeve it where is passes through concrete or brickwork to protect it from the chemicals and acids in the bricks and concrete.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians