Board Thread:General Discussion/@comment-4224349-20140911224512/@comment-26812261-20140914153937

Yeah, I think there's a strong possibility that they're corrupted gems or something. Would explain why we never see any aside from Garnet, Amethyst, and Pearl even though there were clearly many more in the past. Also, Garnet's wording at the end, saying Rose tried to "heal them" instead of saying "tame them", supports the theory.