Blue crystals.. you need to use them on an npc.. check UHall for locations.
Blue crystals: 75k-125k apiece.
Red crystals: 75-125k apiece.
Yellow crystals: 500-700k apiece. Once the halloween event is over.. the yellow crystals will be the only thing left. Search UHall (I forget the post) but the devs stated that only the yellows will be useful after 11/15/9. They make a great guild practice mob.