Interesting question. First, magnets are commonly either an electromagnet created by coiling a current carrying wire or a bar magnet created by a ferromagnetic material. I believe you are referencing the latter. A ferromagnetic material is one where the magnetic properties of the electrons inside the material align with one another when placed inside an external magnetic field. Importantly, ferromagnetic materials stay aligned when the external magnetic field is removed. Iron, a common material, is ferromagnetic. If a rock contains a lot iron or another ferromagnetic material it can become magnetized and create a magnetic field. I highly encourage you to explore this topic further for example this website has interesting facts about magnets click here, please