Saltpeter

Saltpeter is a material from the Railcraft, GregTech and The Vegan Option mods.

Railcraft
In Railcraft, Saltpeter Ore is found just below the surface in hot, dry biomes. It is found in large clusters, and usually has one or two blocks visible from the surface. Saltpeter is interesting in that it will respawn over time, if there is a Saltpeter Ore Spawner somewhere below it, and if there is at least 3 layers of sand above it. Saltpeter Ore Spawners look identical to Bedrock.

The Vegan Option
In The Vegan Option, breaking Sandstone has a 2% of dropping one to three Saltpeter. Saltpeter is used in combination with Sulfur to make Gunpowder, or to make Fertilizer.