In Japanese mythology, the tide jewels-- individually, the kanju (干珠, lit. "(tide-)ebbing jewel") and manju (満珠, lit. "(tide-)flowing jewel")-- were magical gems that the Sea God used to control the tides. Classical Japanese history texts record an ancient myth that the ocean kami Watatsumi 海神 "sea god" or Ryūjin 龍神 "dragon god" presented the kanju and manju to his demigod son-in-law Hoori, and a later legend that Empress Jingū used the tide jewels to conquer Korea. Tide jewels interrelate Japanese dragons and wani sea-monsters, Indonesian mythology, the nyoi-ju 如意珠 "cintamani; wish-fulfilling jewel" in Japanese Buddhism, magic jewels of Nāga kings in Hindu mythology, and the pearl associations of Chinese dragons in Chinese mythology.
Terminology
The Japanese compounds kanju 干珠 lit. "ebb jewel" and manju 満珠 lit. "flow jewel" combine kan 干 (cf. 乾) "dry up; drain off; ebb (tides); recede; oppose" and man 満 "fill; full; rise (tides); fulfill; satisfy" with ju, shu, or tama 珠 "gem; jewel; precious stone; pearl; bead". Compare the reversible compounds kanman 干満 and mankan 満干or michihi 満ち干 meaning "ebb and flow; high and low tides; the tides". Shiomitsu-tama 潮満珠 and shiohiru-tama 潮干珠 are archaic "tide jewel" names using shio or chō 潮 "tide; flow; salt water".