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 likely be made of these new materials.

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.