An elusive gem that radiates light.
The Saintstone is a gem created by fusing two Prismastones together with a Reptite shrine. The party leaves it on the top of Mount Emerald in the Lost Sanctum, where it becomes a Waystone.
The ritual of fusing two Prismastones together carries similarities to the fusion of two Dragon Tears to make the Chrono Cross.
From: Key Items (Chrono Trigger)