The Rime Crystal is a synthesized item in Atelier Meruru: The Apprentice of Arland. After Year 4 when Airshatter awakens at Mt. Velus, Meruru will try to devise some way to stop it from destroying Arls by cooling down the volcano. Totori will give you an idea for the kind of item that might do the job and Hanna will give you ideas about where to find the ingredients needed to create a very cold item. Finally, you can speak to Lord Dessier who will give you the recipe for the Rime Crystal.