In the Battle of Dan-no-ura between the Minamoto clan and Taira clan, the magatama (jewel) sunk in the sea with Emperor Antoku when Nii no Ama, who held the emperor and the box containing the magatama, committed suicide by drowning.
勾玉は、源平の壇ノ浦の戦いにおいて、二位の尼が安徳天皇を抱き勾玉の箱を奉じて入水した。