Sulfur

or Sulfur (Natura). Sulfur is a non-metal added by XyCraft,Railcraft and GregTech. Natura and Thermal Expansion also adds it's sulfur variant, but this version is not interchangable with the others, where the sulfur from the first 3 mods can be used in eachothers recipes.

Sulfur Ore (Added by Railcraft) only spawns in Extreme hills biomes around lavalakes, but it is extremely common there.