The Elemental Crystals are supporting bosses that appear in Super Mario RPG: Legend of the Seven Stars. They fight alongside Culex and compose of a the Fire Crystal, the Earth Crystal, the Water Crystal, and the Wind Crystal.
In battle, each of the Elemental Crystal have their own unique spell to use.
The Fire Crystal, as it name states, is capable of using strong fire based moves such as Flame Wall and Fire Ball. Though it is weak to Ice based moves, it's magical attack is rather high and so is its health but its defense is quite low. It is considered even more difficult to take out than the Water Crystal.
The Earth Crystal has the highest amount of defense out of the four Elemental Crystals, though, it has a rather low magical attack. Its HP is quite high. The Earth Crystal can use deadly earth-based moves such as Boulder and Sand Storm and it's weakness is thunder attacks.
The Water Crystal, even given it's name, only use Ice-based moves such as Blizzard and Ice Rock. It has medium attack status though rather high defenses. The Water Crystal is actually the Elemental Crystal that has the most health of all!
The Wind Crystal is considered the weakest of all the Elemental Crystals. It has one of the lowest defenses in the game, low health, but decent attack. The Wind Crystal uses many thunder-based moves such as Static E! and Electroshock. It can also use the dreaded Petal Blast to turn one of Mario's party into Mushroom.