Many minerals found in nature are naturally magnetic. All normal matter is slightly negatively charged on its surface as a result of the presence of electrons. When these electrons move chaotically, the result is that their slight negative charge is spread out and doesn't demonstrate any clear direction. Such a field is very weak and doesn't generate a detectable magnetic attraction.
Some minerals, such as lodestone, arrange the flow of electrons in a more orderly way. As electrons flow in a general direction, they generate a magnetic field. The greater the proportion of electrons flowing in a uniform direction, the stronger the magnetic field gets.
Iodostone is one of the most efficient natural magnets. It is a black mineral that is rich in magnetite. Magnetite is rich in oxidized iron, and iron generally permits the free flow of electrons through it. This makes magnetite such a powerful magnet that in some animals, such as pigeons, it is incorporated into their brains as an aid to sensing the earth's magnetic field.