Elementine is one of the Unmapped Stars. It glows with a silvery light. It also produces a variation of the fifth element, light in its truest form, Quintessence. The location of Elementine is only known by the Councillors, some Black Swan operatives, Sophie Elizabeth Foster, and Fitz Vacker. Its location is also found on a star map created by Sir Astin, which is one of Mr. Forkle's secret identities. Elementine is known to be explosive and dangerous to use.
Sophie Foster came to know of the location due to the Black Swan’s implanting of the information into her mind. In the third book of the series (Everblaze) Sophie implanted the location into Fitz’s mind so he could assist her in bottling the light from Elementine.
The quintessence from Elementine is the least stable, but also the most powerful, which makes it extremely valuable for creating substances that need to be somewhat explosive or all-consuming, like frissyn.