From Final Fantasy Wiki

Stone III is a tier 3 Black Magic spell in Final Fantasy XI. It deals Earth-aligned elemental magic damage. It has the dubious honor of being the only tier 3 spell that can be purchased from a vendor in the game. This may be because, as usual, Stone III is the lowest level tier 3 spell available to mages in the game. It sells for just under 20,000 gil, depending on fame. It also can be found on several high-level, Black Mage-type beastmen enemies.