From the Super Mario Wiki
Shiny Stone is an item in the game Super Mario RPG: Legend of the Seven Stars. It is obtained from a little mole girl running her own shop in Moleville by trading for the costly Fireworks. It is used as a key to open the sealed door in Monstro Town that leads into Culex's room where the player fights him. It is hinted that no one knows of the stone's purpose, as selling the stone only nets the player two coins. A Shiny Stone can be traded to the mole girl at the shop for a Carbo Cookie.