A powerful Path of Exile currency for crafting.
Fossils was added in the Delve league. It can be placed in special catalysts (with 1-4 sockets)
Fossils can also add special Delve modifiers.

There are currently 36 fossils in poe.

The most expensive one is Fractured Fossil. It can create mirror copies of objects. This allows you to easily and quickly sell expensive items without having to create new one.