Saltpeter is a non-economic yellow stone found within sedimentary layers. It is one of the few stones which will boil in magma.
Saltpeter is also the most lightweight stone besides raw adamantine, so it's preferable for making large pots and absolutely inappropriate for stone-fall traps and catapults.
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In real life, it is used in conjunction with charcoal and sulfur in order to make gunpowder.
It is rumored that if a Dwarf ever discovers this, Dwarven "Civilization" would come to an explosive, but Fun end.
potassium nitrate, good for fertilizer and blowing things up with carbon and sulfur