Bone is a common crafting ingredient that you will require at some stage when you are upgrading your gear. You may find a glove upgrade requires bone before you have found the area. Before you waste time trying to dig up the sand in the desert temple because there is a bone there, that’s not where you find it.
Bone is found by killing skeleton enemies. You will find these in the dark/undead region of the map. If you spot a stone gravestone, hit it with your pickaxe and there is a chance that a skeleton will appear and try to kill you.
Depending on the skills and powers you have unlocked, you may get more than one bone, but typically a skeleton drops a single bone. Once you unlock the temple in this undead region you will come out the other end with more than enough bones to keep you going for the rest of the game.