Pure Heart Crystals appeared in the third season of the Sailor Moon anime series. They took the form of small, pink crystalline objects that seemed to be physical representations of a person's heart. Almost every human's Heart Crystal looked exactly the same, with the exception of Usagi's. There were three Pure Hearts which each contained a Talisman, and these were sought after by the Outer Senshi and the Death Busters.
In episode 102, it was revealed that Pure Heart Crystals would vary in the intensity of brightness based on how pure and genuine the holder was. They were also apparently fragile things and could be easily damaged, as hinted in episode 100.
In the series, when a person's Heart Crystal was taken, they normally either lost consciousness or grew very weak; if it was never replaced, they would eventually die. Because of this, Sailor Uranus and Sailor Neptune often worried about their mission to find the Talismans, since they believed that, because the Talismans were contained within a Pure Heart, the Heart Crystal would have to be permanently removed and its holder would then die.
In episode 94, it was strongly implied that if a person's Heart Crystal had an extended period of absence from their body, they would also die. While a "Pure Heart-less" victim was still conscious, they would become impure as a result of their Pure Heart being taken from them.
Sailor Scouts Pure Heart Crystals