During the Silver Millennium, the Dark Kingdom launched an attack against the Moon Kingdom, ruled by Queen Serenity. The queen managed to trap the Dark Kingdom's greatest warriors, the Seven Great Youma, inside the Silver Crystal, which then fragmented into seven pieces - the Rainbow Crystals.
These crystals were sealed inside the bodies of six humans and one cat, each of whom were eventually tracked down and attacked by members of the Dark Kingdom. Once the crystals were removed from their bodies, the carriers became the one of the monsters of the Great Youma that had been trapped within them. Sailor Moon was able to return them to normal with Moon Healing Escalation.
Queen Serenity imprisoned the Seven Great Youma within seven separate shards of the Silver Crystal, which became known as the Rainbow Crystals (because each one was a different color of the traditional seven-color spectrum). The Crystals were sent to Earth along with Princess Serenity and her court, where the Seven Great Youma were reincarnated inside of normal human beings (and in one case, a cat). The carriers had no knowledge of the Rainbow Crystals inside of them, although in some this manifested as special abilities, such as telekinesis or other psychic powers. While the youma were sealed within the Rainbow Crystals, they were unable to exert their evil influence. Once the crystals were removed from the carriers, however, they reverted back to their youma forms.
The Rainbow Crystals were first introduced in episode 25. Queen Beryl instructed Zoisite to locate the seven crystals in order to re-form the Silver Crystal and fully revive Queen Metalia. To accomplish this task, Zoisite was given the Black Crystal, which could detect and then later remove the Rainbow Crystals from their carriers. During the next seven episodes, a massive search ensued. Zoisite was able to locate each of the Rainbow Crystals and extract them, causing carriers to revert to their Great Youma form. The Sailor Guardians attempted to stop Zoisite, but were usually attacked by the youma. Rather than killing the youma, Sailor Moon used her Moon Healing Escalation attack to seal the youma within their carriers once again.
In Episode 34, the Dark Kingdom managed to collect all seven of the Rainbow Crystals. When Tuxedo Mask was injured while attempting to recover them, Sailor Moon began to cry, and the crystals magically aligned with her. The seven crystals then combined with her tear, and the Silver Crystal was reformed.
- Crane Game Joe - Carried the Red crystal.
- A priest - Carried the Orange crystal.
- Ryo Urawa - Carried the Yellow crystal.
- Yumemi Yumeno - Carried the Green crystal.
- Reika Nishimura - Carried the Blue crystal.
- Grandpa Hino - Carried the Indigo crystal.
- Rhett Butler - Carried the Violet crystal (only non-human carrier, a large tomcat).
- The Rainbow Crystals have a similar appearance to the Chaos Emeralds in the Sonic the Hedgehog video games. However, the seven Chaos Emeralds are much larger in size, and the coloring is also different; the Chaos Emeralds feature a white-colored piece instead of an indigo-colored one, and a cyan-colored piece instead of an orange-colored one.