Crystal is a Green-Jeweled Queen, and an earth elemental.  Despite her Jewels, tho, she is the undisputed leader of the coven.  That's what Queens are born to do.

Another thing Queens are born to do is heal the land.  That's why I made her an earth elemental.  Also, note her circlet?  The black stone there?  It's actually a chip of Mordred's Black Jewel, the darkest (read: most powerful) Jewel there is.  It's the way Mordred is able to control Crystal for his own ends.  --Mina, 03.5.21