Small "clouds" of native mercury appear to float around Cinnabar at all times. In the same way that mercury is harmful to human beings (due to its toxicity) and destructive to metals such as aluminum (due to forming amalgams that compromise their strength), Cinnabar's very presence is hazardous to their fellow Gems. As a result, they have been forced to live a mostly solitary existence, well removed from the "school" where the rest of the Gems live.
Known as "Shinsha" in Japanese. Kongou Sensei and Shinsha are the only Gems who use Japanese names instead of English in the original Japanese version.