Magnetite

Magnetite is the only mineral that exhibits strong magnetism, although others like ilmenite, chromite and hematite may have weakly magnetic behavior. Magnetite has a Mohs hardness of about 6 and a black streak. Most magnetite occurs in very small grains. A chunk of well-crystallized magnetite like the round specimen is called a lodestone. Magnetite also occurs in well-formed octahedral crystals like the one shown.

Magnetite is a widespread accessory mineral in iron-rich (mafic) igneous rocks, especially peridotite and pyroxenite. It also occurs in high-temperature vein deposits and some metamorphic rocks.

The earliest form of the sailor's compass was a rod of lodestone mounted on cork and floating in a bowl of water. The rod aligns with the Earth's magnetic field to point roughly north-south. Magnets hardly ever point exactly north, because the geomagnetic field is tilted relative to true north, and moreover it slowly changes direction over time spans of decades. If you're navigating at sea, it's much better to use the stars and Sun, but if those are not visible, then the magnet is far better than nothing.

Other Primary Minerals
Other Hydrothermal Vein Minerals
Other Metamorphic Minerals