Bound fossil belongs to the class of stackable currency. If placed into the resonator, this one of fossils can affect item crafting. Bound adds more aura or minion mods. The spawn weights of mods with minion tags are multiplied by 10 and aura ones by 10. In Path of Exile game the location where it can drop is Azurite Mine namely Petrified Forest and Abyssal Depths. It is also possible to buy POE Bound fossil for 10,000 azurites from one of 5 masters, Niko the Mad, or exchange it for 3 Chaos orbs.