Saltpeter is a crafting component that was added in a May 29, 2016 update. It is an essential component when making Flintlock Ammunition, requiring one unit of Saltpeter for every piece of Gunpowder made. Saltpeter is only gathered when using a Silver, Gold, or Gemstone based pick on any of the Non-Spawn islands, practicly replacing coal as a typical drop in these conditions.
Let it be known that Saltpeter is different from Salt. One is a crafting component, and the other is a material. Keep this in mind when trading with other players; It might save you from getting scammed! Saltpeter is uncommon and worth 90 - 100 doubloons each. Salt and Saltpeter share the same item icon.