Master Materia
There are three different Master Materia: Command, Magic, and Summon. They can each be gained by taking mastered versions of regular materia to the Huge Materia in Cosmo Canyon and trading them in. The goods are lost, but a Master orb is given in return.
These materia function as compressed copies of each regular materia. That is to say that equipping Master Magic provides all magic spells. They may also be used with support materia, in which case it is treated as if each regular materia is linked to it.
Master Materia cannot be leveled up, nor can it give birth to new materia. The only way to acquire more is to trade in more mastered orbs.
