The Keystone is a Materia needed to open the "door" in the Temple of the Ancients. A man living in a house near Gongaga Village had it, but gave it to Dio. Cloud makes a deal with Dio, and fights in the Battle Square for it. He eventually receives it, but because the tram down to North Corel has broken down, the party has to spend the night in the Ghost Square.
During the night, Cait Sith steals the Keystone and hands it over to the Turks. Cloud reluctantly continues on, as usual. When the party arrives at the Temple of Ancients, they find Tseng, badly wounded by Sephiroth. Tseng gives the Keystone back to Cloud, who uses it to enter the temple.