If Materia orbs are inserted into slots in weapons or armor, various abilities become available. By equipping certain Materia, a character's Strength, Magic, Max HP, or Max MP, may change. You can level up materia to learn new abilities by gaining AP (ability points) earned by winning battles. Materia that is not equipped on weapons or armor will not obtain this AP.
Various types of Materia appear in the game. Materia can be divided into five groups based on their Abilities.
Purple Materia. This enhances the character's Status, such as maximum HP and strength.
Blue Materia. This Materia shows full effect when used in conjunction with other Materia, such as "All" or "HP Plus". Make sure you insert this Materia in linked slots when equipping.
Yellow Materia. This will increase the number of commands that can be used during battle, such as "Steal" or "Throw"
Green Materia. This will enable you to cast a number of Attack Magic and Curative Magic.
Red Materia. This will enable you to cast Summon Magic during battle, in order to call a Summon Monster.
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 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)