Imperial regalia; sacred jewel
Specifically refers to the Yasakani no Magatama, one of the three Imperial Regalia of Japan. Used in historical, ceremonial, or mythological contexts.
See also: 八尺瓊勾玉
The sacred jewel is one of the three Imperial Regalia.
emperor's seal
Refers to the imperial seal used by the emperor. A formal, historical term; not used for ordinary seals.
The imperial edict was stamped with the emperor's seal.