Ruins

Ruins are a possible addition to the 1.8 "Adventure" update. They are a new type of structure that seems to occur naturally ( Stone Brick, Mossy Stone Brick, Cracked Stone Brick, and Iron Bars were revealed in the same Image, posted by Jeb on his Google+ account on July 1st 2011. The ruins will be made of these materials most likely

Although they appeared in the screenshot, they are not 100% confirmed as of yet, as they have not been named or even mentioned by any of the developers.