They were separate sections of the Silver Crystal, which came apart when Queen Serenity sealed away the Dark Kingdom's greatest warriors, the Seven Great Youma, during the attack on the Moon Kingdom in the Silver Millennium. They were sealed into six humans and one cat, each of whom were eventually tracked and attacked by members of the Dark Kingdom. Once the crystals were removed from their bodies, the carriers became the Great Youma they had the connection with. 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 Rainbow Crystals because each one was a different color of the rainbow. The Rainbow Crystals were then sent to Earth, where the Seven Great Youma were reincarnated inside of normal human beings (and in one case, a cat). The humans (and cat) had no knowledge of the Rainbow Crystals inside of them, although some had special powers such as telekinesis or psychic powers. Because the youma were sealed within the Rainbow Crystals, they were unable to exert their evil influence. Once the Rainbow Crystals were removed, however, the humans reverted to their youma forms.
The Rainbow Crystals were first introduced in episode 025, in the first season. Queen Beryl instructed Zoisite to locate the seven crystals in order to form the Silver Crystal and fully revive Queen Metalia with its power. During the next seven episodes, a massive search ensued. Zoisite was able to locate the Rainbow Crystals by using the Black Crystal. He would locate one of the Rainbow Crystals and extract it from its carrier, causing the person to revert to his Great Youma form. The senshi 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 its carrier once again.
In episode 034, the Dark Kingdom managed to collect all seven of the Rainbow Crystals. However, when Tuxedo Mask was injured, Sailor Moon began to cry, and the crystals floated to her instead. The seven crystals then joined with her tear in order to form the Silver Crystal.
- The crystals look similar to the Chaos Emeralds in the Sonic the Hedgehog series. And coincidentally, there are also seven Chaos Emeralds. However, the Chaos Emeralds are much larger in comparison. The coloring is also different. The Chaos Emeralds feature a white-colored piece instead of an indigo-colored one, and instead of an orange-colored piece, the Chaos Emeralds have a cyan-colored one.