iPhone 5 64GB Warm When Playing Games

Hi. When I play games that are a little graphically intense, it becomes warm. It's even really warmer (but not hot that I can't hold it) when I charge it up while I play games from companies like Cave Ltd. (which makes bullethell type shooters which have pretty intense graphics, lots of going in the game). Although I've heard that it's warm because it's aluminum which dissipitates heat better than the other past iPhones. Will this damage the iPhone 5 though? Or Apple's designed it well enough to be ok with that much heat? My iOS is the latest, 6.0.2.

By the way, how would I know if my iPhone's not a defected one. Is there an app for diagnostics? And an app to test it's battery if not a bad battery?

Thank you on advance. Happy new year.

God bless.