Stone

 is a basic part obtained by using a pick on Stone Deposits or found scattered around on the ground at the start of a new map.

If there are no stones left on the ground in any areas the player has explored, one will spawn next to a random boulder (if there are any boulders in explored areas) after 10 seconds.

Use as a tool
Although multiple stones can be carried at a time, you must be holding a single stone to use it as a tool.