As Lord Golbez says, all of the FFs have had a crystal element. VIII had Lunatic Pandora which encased a massive crystal that summoned the Lunar Cry. IX always had the crystals for the summons as a backstory in Madain Sari besides the World Crystal that appears at the end. X has the spheres and XII has nethicite. I'm not sure if there is an FF title that doesn't have some plot important crystal element.