In Animal Crossing: New Horizons, there are various materials you’ll need for crafting furniture and tools, such as Iron Nuggets, Clay, and Gold Nuggets.
All three materials could be found by hitting a rock with an Axe or a Shovel.
Iron Nuggets and Clay are one of the common drops from rocks and they respawn on a daily basis.
Here’s a video showing how it works:
Gold Nuggets, on the other hand, are very rare and hard to come by.
So if you receive one you should keep them safely until you need them for crafting or selling it for a lot of bells.