Keys...........keystones........the Francis Scott Key is made of a type of rock............black canisters.....near the DC wide......on the Teddy R. island side.......explosives???

As Langdon explains, ... But as Langdon and Sophie discover, the keystone is more of a metaphor than previously thought (like everything else in Da Vinci Code), and is in actuality the cryptex, a device cleverly crafted by Saunière.