Key to the Temple of the Ancients.
Description
The Keystone (キーストーン, Kīsutōn?) is a key item in Final Fantasy VII and Final Fantasy VII Rebirth. 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.
History
Original continuity
Cloud made a deal with Dio in "Secret Date" and fought in the Battle Square for the Keystone. He received it, but because the tram down to North Corel broke down, the party spent the night in the Ghost Square. During the night, Cait Sith stole the Keystone and handed it over to the Turks. When the party arrived at the Temple of Ancients, they found Tseng, badly wounded by Sephiroth. Tseng returned the Keystone to Cloud, who used it to enter the temple.
Remake continuity
After learning that Dio had the keystone in his collection, and that the black materia the Gi tribe had made was now held by Cetra magic in the temple, the party headed to the Gold Saucer. Cloud and his friends partook in the Naming Rights tournament at the Musclehead Colosseum in "A Golden Key", but Cait Sith ultimately took it and evaded the party's search through the amusement park long enough to hand it over to Tseng who left on a helicopter. The party followed the Turks to the temple's location regardless, and found the Turks had already used the keystone to manifest the temple.