Saltpeter is a non-economic and low-value yellow stone found within sedimentary layers. With a melting point of 10601 °U and a boiling point of only 10720 °U , it is one of the few stones which will boil in magma.
In real life, it is used in conjunction with charcoal and sulfur in order to make gunpowder. This process does not exist (yet; it seems Toady has plans to include gunpowder later, but only for simple uses) in Dwarf Fortress, and therefore gunpowder is also non-existent.
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It is rumored that if a dwarf ever discovers saltpeter's capabilities, dwarven "civilization" would come to an explosive, but Fun end.
potassium nitrate, good for fertilizer and blowing things up with carbon and sulfur
[SOLID_DENSITY:2105] Previous value was much too low