After you've escaped the Leviathan you have the opportunity of returning to the Giza Plains and making more sunstones. To trigger this go speak with Masyua in the Nomad Village. She will give you a Shadestone. Then it's time to venture out into the plains to charge the shadestone. Unlike the first time only two stones will be active at a time. They should be marked on your map with an exclamation point. Keep going to active crystals until your shadestone becomes a sunstone. Return the stone to Masuya for a reward of 200 gil, 2 Potions, and a Holy Stone. This task can be performed an unlimited number of times until the end of the game.