A Magatama is a style of Japanese beads shaped as a circle and flowing tail that first appeared around 14,000 BC to 300 BC. Their popularity continued throughout the ages, and they have become a popular item used in manga and anime.
In the fanservice fighting series Ikki Tousen, the Magatama jewels worn by all those consider themselves true fighters, the essence of the fighters strength. The jewel starts to change colors, as the fighter grows stronger and defeats more opponents.
In the murder-mystery series Phoenix Wright, the magatama are jade jewels that are used by the mystics of the Fey family. One was given to Phoenix Wright to help him to unlock the secrets people are withholding during his investigations.