The Amethyst is located in a small, underwater cavern in Soda Seas beneath the giant puffer fish. To retrieve it, you must find all the cream soda barrels and then go down to the bottom. Then, you must get past the four small puffer fish. Then, you must sneak around the permanently inflated regular puffer fish, who are circling the giant puffer fish. It is thought to be the most difficult non-secret treasure to retrieve in the the game.
It is a purple, jagged crystal that is sometimes embedded in a rock.