Board Thread:General Discussion/@comment-27464043-20151230192612/@comment-28278649-20180410140842

Not the Eternal Flame, as it perished with Surtur. Besides, the movie already had two Infinity Stones in it, what's the point in having one hidden in plain sight? That would be a retcon and would need to be explained in the film.

Also, considering all it did was give Surtur a slight power boost and, it's not strong enough to be an Infinity Stone.