Visions of Mana

Corestones will sometimes be earned after battle with monsters. For some reason Val has a special ability to turn monsters to stone, so you'll get their corestones after a battle randomly. You should easily get 30 of these naturally when playing through the story, as bosses and nemeses seem to be guaranteed drops as well. From any item shop you can then exchange these stones for ability seeds. They have no other purpose aside from possibly getting Niccolo Coins (which have very little purpose of their own), so exchange them whenever possible for better ability seeds.