Final Fantasy VII/Magic Materia

The magic materia is green. Usually, it has 3 to 5 levels (stars), and each time the materia gains a level, it's possible to use a new spell, with the first being equivalent to slight tickling, the second being the one you use most, and third one being the one that kills everything dead (or heals everyone alive or something). There are a few exceptions like Ultima. The first star filled at the beginning and the last star being filled means the materia is mastered, thus depending on the stars the materia can usually have 2 to 4 spells - in Ultima's case, only one.

The materia is here divided quite arbitrarily to Black and White magic; this division is more of a traditional Final Fantasy thing and isn't used in the games as such. Also, the Black magic is divided to Elemental and Non-Elemental attacks.

Gravity
The Demi spells cause a lot of damage (particularly to fliers), but are not too accurate. Also, most bosses are nearly immune to Gravity effects as well.

Effective against Emerald Weapon when used with W-Magic or Quadra Magic.

Comet
Can be obtained from the top floor of one of the houses in the Forgotten Capital.

Contain
Multi-materia attacks.

Can be obtained from Mideel. Give it greens (Mimett Greens) to the small white chocobo, then scratch it behind the ears.

Ultima
Can be obtained for free by stopping the materia train in Corel in Disc 2. If you fail to stop the train, it can be bought.

Full Cure
Full Cure materia is obtained from one of the houses in Cosmo Canyon, in Disc 2 or later.

Master Magic
The Master Magic materia basically gives you all the abilities of all the magic materias. It can be gotten either by defeating Emerald WEAPON and trading in the Earth Harp at Kalm, or by mastering all the magic materias and trading them for the Master Magic materia at Cosmo Canyon (if you managed to save that Huge materia anyway). The Master Magic materia can be linked to and "All" or a "Quadra Magic" materia to give all spells the effect of the support materia.