You might be looking for Keepers of the Lost Cities Book Three: Everblaze.
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This article contains plot details from the latest books.
Everblaze is an unstoppable flame that burns a neon yellow color. Even Pyrokinetics cannot truly control it. It is very dangerous, and the only thing that can stop it is a formula called Fryssin, which contains Quintessence, the light of an unmapped star. Sophie bottles the Everblaze with Lumenite and gold, in a short, round bottle with a fluted neck and a wide opening, in comparison to other fire-catching bottles, which are long and thin. In book three, Fintan ignites the Everblaze when Sophie heals his mind. Before enough Fryssin is created, it kills Kenric and destroys a lot of Eternalia.