Final Fantasy VII

Master Materia

To get the three Master Materias, beat Emerald Weapon and you will get the Earth Harp and go to Kalm and give it to a Kalm Trader who will give you these the three master materias.

There is another way to get the master materias, so if you do not feel up to defeated Emerald Weapon (which is an optional battle), you can do the following:

You can master all the materias of that type and talk to the Huge materias of the respective color at the Cosmo Canyon observatory. You can have any number of Master materias this way. You can master Command (yellow materia), Summon (red materia), Magic (green materia)