Not to be confused with salt.
Saltpeter is a crafting component that was added in a May 29, 2016 update. It's an essential component when making Flintlock Ammunition, requiring one unit of Saltpeter for every piece of Gunpowder made. Saltpeter is only gathered when mining on any of the Non-Spawn islands, replacing coal as a typical drop in these conditions. Salt and Saltpeter have the same icon, but Salt is currently only used in cosmetic applications. It is often used to scam players into thinking they will get saltpeter.
Saltpeter can also be collected via Smokey, located on the Unnamed Island south of Nova Balreska.