Frissyn is a shimmering, silver powder made from Sasquatch saliva, Quintessence and a number of other ingredients. It is the only thing that can extinguish Everblaze, also known as the unstoppable flame. The substance is extremely cold, as remarked by Sophie.
In Book 3, after Fintan's attack of Everblaze at Oblivimyre, Sophie and Fitz were instructed to create Quintessence at the last minute to stop the fires from destroying too much of Eternalia. Sophie is one of the only elves to know the formula, which was implanted into her brain (presumably by Mr. Forkle or another elf in the Black Swan).