One of the most natural way to make a rock background is to use real rocks. Many people have made backgrounds using real rocks and they have turned out awesome! Careful with real rocks as they do make a very heavy tank background!
Here are some photos of some tanks that utilize real rocks as there background!
106_1791 by nickerson_kevin, on Flickr
106_1793 by nickerson_kevin, on Flickr
106_1821 by nickerson_kevin, on Flickr
106_1850 by nickerson_kevin, on Flickr
Here is a build by dendroboard user Cairo11 that utilizes lava rocks! This would make it not quite as heavy as the previous example.