Main Story:
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 The main story of Final Fantasy VII Explains that the Main function of WEAPON is to protect the planet from it's enemy that hurts the planet and bring ill to the planet..

The Main story has 4 WEAPON..
-Sapphire
-Ruby
-Diamond
-Emerald

In the Sequel of the original Final Fantasy VII, Final Fantasy VII: Dirge of Cerbrus, there  is one more WEAPON to introduce, and that is Omega.