Purchased from Vuo Nin the Barkeep at The Pine and Fir tavern in Lin.
A Sparking Stone
I've heard tell of a strange purple stone in a cave south of Lin and west of Mayoi. It's guarded by nasty zefirs. There's also a book there, but no one I know has actually braved the zefirs to see what it's all about. If you want to try your luck, head northeast from the West Mayoi Mountain Villas.
-- A Sparking Stone