Brimstone

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late Old English brynstān, probably from bryne ‘burning’ + stān ‘stone’.

wiktionary

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From Middle English brymston, brimston, bremston, forms of brinston, brenston, bernston, from Old English brynstān(“brimstone”, literally “burn-stone”), equivalent to brian +‎ stone, or burn +‎ stone. Cognate with Scots brunstane(“brimstone”), Icelandic brennisteinn(“sulfur / sulphur, brimstone”), German Bernstein(“amber”). Compare also brimfire. More at burn, stone.

Once a synonym for " sulfur / sulphur", the word is now restricted to Biblical usage.

etymonline

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brimstone (n.)

"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 (see stone (n.)). 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.

The Lord reynede vpon Sodom and Gomor brenstoon and fier. [Wycliff's rendition (1382) of Genesis xix.24]

The Old Norse cognate compound brennusteinn meant "amber," as does German Bernstein.