The wear bar
The bar under a tool's name goes from green to amber to red. When it runs out, the tool breaks in your hand. Stone wears out fastest, copper lasts longer and iron longest. Light blows wear a tool less, and the wrong material wears it faster. Keep a spare.
Replacing and recycling
Make stone tools on the Pack tray and copper and iron ones at the Forge. Drop an old tool in the Recycler to get some materials back.