Amarallitine, a highly combustible 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 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. The solution that Dex and Kesler were creating was "the first one [they] hadn't exploded all day."
Elwin also mentioned working with Amarallitine in 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.