Board Thread:General Discussion/@comment-4224349-20140911224512/@comment-25234730-20140915221319

They might be Gems that made the ultimate taboo to Gemkind. The rule that Gems must never harm Humans (or any other sentient life). A Gem that kills Humans gets tainted and if they continue this murderous act, will eventually turn into the very monsters they are.

I also believe that the ancient Gem war could have caused the Gem's population to dwindle down to near extinction.