Символ штата Пенсильвания - замко́вый камень.
http://en.wikipedia.org/wiki/Pennsylvania
Pennsylvania has been known as the Keystone State since 1802, based in part upon its central location among the original Thirteen Colonies forming the United States, and also in part because of the number of important American documents signed in the state (such as the Declaration of Independence). It was also a keystone state economically, having both the industry common to the North (making such wares as Conestoga wagons and rifles) and the agriculture common to the South (producing feed, fiber, food, and tobacco).
http://en.wikipedia.org/wiki/Keystone_%28architecture%29
A keystone is the wedge-shaped stone piece at the apex of a masonry vault or arch, which is the final piece placed during construction and locks all the stones into position, allowing the arch to bear weight.