With each new “Final Fantasy” game, there are always mechanics and new content that differ from previous games of the series and each make the particular game more unique and different from the last or even the game that will follow it. This is no different for “Final Fantasy VII”.
One of the mechanics that makes “Final Fantasy VII” stand out from it's predecessors is the way the game revolves around an element called Materia. In the world of “Final Fantasy VII”, Materia is something created from the Planet's energy and commonly manifested in the form of a small, usually marble or tennis ball sized, stone. Depending on the Materia's abilities and effects, each stone varies in color, and the more powerful the Materia is, the larger the stone will be.
Each character has a certain amount of 'slots' to equip Materia in, and these slots are determined by the character's weapon as well as an accessory. In addition, certain Materia slots are linked with others, so one can combine certain Materia to create different effects. As the game progresses, generally when a character receives a new weapon it comes along with more Materia slots, though depending on the power of the weapon one might be left with less or even no slots to place Materia into.
Each weapon also provides a sort of 'growth' for the equipped Materia. The standard is simply 1x growth, but with certain weapons it is possible to get “double” and even “triple” Materia growth. This growth is useful because as the player completes battles, in addition to other rewards at the end of battle, “AP” is given out, and each Materia has a certain amount of AP needed to gain a star. Each Materia usually has three to four stars, though there are certain pieces of Materia that only have two stars. As the Materia gain stars, the magic or abilities become stronger. Upon gaining all stars for a piece of Materia, it will duplicate itself and the player will receive a second of the same Materia, the new copy having only one star and requiring more AP. This is extremely useful for certain Materia that is extremely rare and can only be found once in the game.