The first recorded usage of the word brimstone in the early 14th century and was used to describe what we now know as Sulphur (or Sulfur). The word means “burning stone” or “the stone that burns”.
“sulfur in a solidified state,” Old English brynstan, from brin– stem of brinnen “to burn” (from Proto-Germanic *brennan “to burn,” from PIE root *gwher- “to heat, warm”) + stan. … In Middle English the first element also recorded as brem-, brom-, brum-, bren-, brin-, bron-, brun-, bern-, born-, burn-, burned-, and burnt-. Formerly “the mineral sulfur,” now restricted to biblical usage.
Around the area of the Dead Sea, deposits of Sulphur would have been common during biblical times. According to the International Standard Bible Encyclopedia “That the inhabitants of the land had experienced the terrors of burning sulphur is very probable. Once one of these deposits took fire it would melt and run in burning streams down the ravines spreading everywhere suffocating fumes”. Considering this, its biblical association with Hell and fire is understandable.
Sulphur is the fifth most abundant element on earth and the tenth most abundant in the universe. It is found in many common minerals (e.g. gypsum and pyrite).
Bonus Biscuit: What does sulfur smell like?
Rotten eggs, right? Nope.
Compounds of Sulphur (like Hydrogen Sulphide) smell like rotten eggs, but Sulphur itself is completely odourless.