Fossils are natural cubic blocks that can be obtained by mining, looting, pet-harvesting or forging. They can be used for crafting, processing, building and decoration.
How to obtain
Like most natural blocks, these blocks do not spawn after world creation, but are part of the pre-generated template Worlds that each new game world is a copy of. Whenever a player creates a new world (like by starting the game for the first time), one of the currently 4 template Worlds is randomly chosen and copied to become the new player game world.
Occasionally, blocks of Fossils can be obtained from Warmworms and Dustevils, either when these Creatures are killed or harvested from when they are Pets. By that, Fossils are an infinite resource; but they can also be bought in a theoretically infinite amount via Blueprint Kits like all other placeable blocks and objects.
Mining these blocks does not require any Power Cell to be equippped since R39 in February 2017 anymore.
Obtaining Fossils is one of the unlocking requirements for the crafting recipe for Asphalt.
How to use
Fossils can be placed just like they are for decoration or as building blocks, however when placed on the ground and not lit by artificial lighting, they can make Mirus, Night Rocksters and/or Night Pigsies spawn in darkness (like at night); rarely also Stone Treasure Chests.
Other than that, Fossils can also be put on display by placing them into the slots of display containers like Placemats, Stone Wall Shelves, Hidden Temple Altars, Galactic Grav-Chambers, Frozen Containers, Holiday Decorative Trees, Snow Buried Containers or the like, where they will usually be reduced in size while on display and will not be able to spawn Creatures or Treasure Chests.
The Quest "No Block Unturned" (rewarded by 10 Healing Beacons, 10 Corruption Resistance Potions, 10 Health Regeneration Potions and 2 placeable Diamond Treasure Chests) requires to discover blocks of Fossils (by obtaining one of these blocks), but also to discover Glowing Mushrooms, to discover blocks of Sulfur on the Lava layer and to discover blocks of Corrupted Leaves (that require at least a Diamond Mining Cell to be mined).