They're very rare items that allow you to get the best Armour in the game. You need to use a Siren in one of the rooms of the Lunar Subterrane, and then engage in a very weird battle with some Flan Princesses and hope to God that you get a pink tail afterwards. If you do end up with a pink tail in your inventory, you'd then have to go to the Adamant Grotto and give it to some guy and recieve the Adamant Armour.