"It was brimstone. A weal of brimstone all about the rim of the caldron, bright yellow and shining here and there with the little flakes of silica but most pure flowers of sulphur."
Sulphur on the rim of Pacaya volcano's crater, Guatemala
Creative Commons Attribution Share AlikeSulphur on the rim of Pacaya volcano's crater, Guatemala - Credit: Bruno Girin

Brimstone is an alternative name for sulphur.

Being abundant in its natural form, sulphur was known in ancient times and is referred to in the Torah and the Bible, where English translations commonly referred to burning sulphur as ‘brimstone’. Its association with volcanic activity probably informed the ‘fire and brimstone’ conception of Hell.