Key to the Temple of the Ancients.
The Keystone (キーストーン, Kīsutōn?) is a key item in Final Fantasy VII. It is a Materia needed to open the "door" in the Temple of the Ancients. The Weapon Seller living in a house near Gongaga had it, but he gave it to Dio, the owner of Gold Saucer.
Cloud makes a deal with Dio and fights in the Battle Square for the Keystone. He 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, and 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.