Amarallitine, a substance used in alchemy, is described to come in a shade of luminescent yellow. It is described by Elwin as volatile, and is kept in a vial. When it was mentioned in Book 1: Keeper of the Lost Cities, Dex used a pair of quite lengthy tongs to handle it. In preparation for the reaction it can cause, Kesler also put on a pair of thick black glasses. In the solution the Dex and Kesler were creating, it exploded after this substance was put in.
Elwin also mentioned working with Amarallitine in Book 8.5: Unlocked, saying that as it was a volatile substance that he was not used to working with, he would need to consult Kesler for help.