After defeating Calbrena and Golbez in the Dwarven Castle, you will get Rydia back. When King Giott finishes speaking to you, cast the Warp spell. You will appear in the Crystal Room behind the throne and the Crystal is strangely still there! Take it and this will save you the trip to the Sealed Cave. You will still have to take Luca's Key and enter the exit the Cave, but then Kain will take the Crystal as if you defeated the Evil Wall for it.