There is a special kind of Materia known as Master Materia. This Materia can be for Magic, Command, or Summon commands, and contains the effects of all the Materia of that type in one. You can obtain two of each.

Unlockable	How to unlock
Master Command Materia	Master all Command Materia.
Master Command Materia	Defeat the Emerald Weapon, and give the Earth Harp to the traveller in Kalm.
Master Magic Materia	Master all Magic Materia.
Master Magic Materia	Defeat the Emerald Weapon, and give the Earth Harp to the traveller in Kalm.
Master Summon Materia	Master all Summon Materia.
Master Summon Materia	Defeat the Emerald Weapon, and give the Earth Harp to the traveller in Kalm.