Magatama were curved beads made from semi-precious stone (jasper, agate, etc.) used in various Shinto rites. They appeared somewhere around the Jomon period and maintained their popularity until the Nara period, when they were largely eclipsed by Buddhist Prayer beads.
Usage in the Ace Attorney Series
The magatama featured in the games is owned by Maya Fey, but is used by Phoenix Wright. During Phoenix Wright: Ace Attorney: Justice for All, Maya's sister Pearl imbues it with spiritual power. The Magatama is used in the course of Phoenix's investigations to allow him to detect when others are lying or covering something up . The lies are visualized as a series of chains and bright red locks (called Psyche-Locks), and are broken when Phoenix presents evidence to disprove his opponent's version of events. The Magatama has a health bar attached to its use, therefore Phoenix can only make a few wrong choices before he is forced to quit. A successful Psyche-Lock break restores up to half of Phoenix's health bar.
A similar mechanic is used by Apollo Justice in Apollo Justice: Ace Attorney, which uses his innate observational power to detect a person's nervous twitches when they lie.
Shin Megami Tensei: Nocturne
Magatama appear in Shin Megami Tensei: Nocturne as small, metallic parasites that play an important role in customization.
Magatama are to be equipped one at a time and offer the player a wide array of skills to pick and choose from upon leveling up. Skills can be acquired once the player reaches a certain level, though the necessary level required is never explicitly stated. However, one may notice the Magatama icons writhing violently on the select screen. The more restless a particular Magatama seems, the closer it is to acquiring it's next skill. Occasionally an equipped Magatama will react violently upon leveling up, with effects ranging from an increase in stat points to a number of random status ailments.
Each Magatama is named after some figure or location from various mythologies (Muspell, Gehenna, etc.).