A block of Potomac Sandstone -- originally part of the U.S. Capitol in Washington, D.C. -- embedded in the exterior wall of the Katharine Loker Center at the Richard Nixon Presidential Library and Museum in Yorba Linda, California.
Potomac sandstone is also known as "Potomac bluestone" and as Aquia Creek sandstone. It is quarried at Government Island, Virginia -- about halfway down the Potomac River to the ocean. Potomac sandstone is known as the "rock that built the capital" because it was widely used during the construction of most early government buildings in Washington, D.C.
Potomac sandstone is not a high-quality stone, however. It is very coarsely grained, and contains extensive amounts of pebbles and clay in it. It is soft and easily carved, but it also does not stand up well to weathering or pollution. Initially, it has a bluish cast to it -- but this quickly goes away once it is exposed to air, sun, heat, and rain.
This particular block of Potomac sandstone was donated by the U.S Congress to the Nixon library. It was presented on President's Day in 2003, during the groundbreaking for the Loker Center.
The Loker Center comprises the southern wing of the Nixon museum. Completed in 2004, it almost doubled the size of the museum. It contains a 184-foot long corridor, a massive exhibit hall, two small meeting rooms, a catering kitchen, and an exact replica of the East Room of the White House (which serves as an event venue for the library).
During construction of the Loker Center, the museum entrance hall was enlarged to 1,350 square feet, a new Annenberg Center pavilion added as a ceremonial entrance to the museum and library, and a new research library and archival storage space built underground below the Annenberg Center and the museum entrance hall.
Construction of the Loker Center essentially completed the Nixon Library and Museum.