Brimstone is a yellow stone found small clusters in igneous extrusive layers as well as within gypsum. It is neither fire-safe nor magma-safe.
Brimstone is a name for a native crystalline form of the element sulfur.
Brimstone is really just elemental sulfur, but it sounds cooler
Annotated image, click for details
Sulfur burns with a blue flame, and this creates a blood-red liquid
A human carrying slabs of sulfur
brimstone is a better name for sulfur than sulfur, but this is just sulfur