Rainbow Shell
The Rainbow Shell can be found in Giant's Claw in 600 AD after the defeat of the Rust Tyrano. Due to its size and weight, however, it cannot actually be lifted by your party (not even Robo is able to lift it), and is thusly never actually added to your inventory. It is, however, carried back to [[Guardia Castle] by the King's knights as a form of repayment for all your party had done to assist them.
The Giant's Claw is an optional side-quest. The side-quest is completed when the party requests the shell to be moved and stored until the year 1000 AD. Additionally, the completion of this side-quest opens a second side-quest, dealing with the trial of the King in the year 1000 AD.
Upon completion of the Trial side-quest, the Rainbow Shell can be forged by Melchior into either three Prism Helms or one Prism Dress.