Heart-shaped necklace

From Inga no Kusari wiki

Heart-shaped necklace is Ada Akiyama's glint. This necklace was given to Ada as a gift from her 'father'. The necklace is sparkling silver, the chain being very thin. A heart-shaped diamond piece is connected to the necklace, and although it's not melded on, it cannot be removed. The diamond, when certain conditions are met, becomes a black diamond. This strange heart-shaped necklace has been altered unconsciously by Ada's innermost thoughts and emotions, the positive, as well as the twisted inner workings of her spirit. When white, it reflects the purity of her inner self; When the diamond becomes corroded and turns into a black diamond, however, Ada's own inner darkness overcomes her.

Glint Ability

The diamond necklace reflects which side of Ada is currently 'dominant', and therefore in control of her body. While Ada has no control over the constant switching, it serves as a warning to those who know it's power; When it's a pure diamond, Ada's normal persona, bright and cheerful, is the dominant one. When it's black, however, Ada's darker persona is dominant.