Archaeologists in Japan have uncovered what may be early Japanese ninja weapons including rocks that may have led to the metal throwing stars beloved by Hollywood and pre-teens. Uncovered during excavations at Iwatsuki Castle and Hachioji Castle, the flat "throwing stones" and clay caltrops may date back to the Siege of Odawara in 1590. From Live Science:
The siege took place during the Sengoku period (1467-1615), a time when Japan was divided between several warlords who battled for power. Historical texts mention ninjas as spies and saboteurs during this time and they likely took part in the siege.
The artifacts include flat throwing stones that may have been the predecessor of the shuriken throwing star and clay caltrops that may be an early form of the makibishi caltrop — a spiky weapon that could injure the feet of soldiers and horses. These artifacts were likely the weapons of a "battle group which can move into action as ninjas," Iwata Akihiro, an archaeologist and curator at the Saitama Prefectual Museum of History and Folklore, told Live Science in an email.