Nitre

Saltpeter is a slightly rare mineral that can be found by mining Saltpeter ore . Combined with Sulfur and a charcoal (can't be coal) in a crafting table it gives 2 gunpowder. This is the only way you obtain gunpowder in Middle Earth to make things such as Orc Bombs.

Saltpeter can also be used to make bonemeal by crafting it together with dirt.