Mana Crystals are crystals used by Keidran and, with harmful effects on the user, by Basitins, to perform magic. Other than draining their own life force via Black Magic, it is the Keidran's only source of mana.  It seems that lower-quality ones crack and become useless after prolonged use.

Image of General Alabaster with a necklace of mana crystals is from comic 505.