Magnetite is a mineral and one of the main iron ores. With the chemical formula Fe3O4, it is one of the iron oxides. Lodestone is ferrimagnetic it attracts a magnet and can be magnetized to become a permanent magnet. It is the most magnetic of all natural minerals on Earth. Naturally magnetized pieces of lodestone, called mudstone, will attract small pieces of iron, which is how ancient people first discovered the property of magnetism. Today it is mined as iron ore.

Small grains of magnetite occur in almost all igneous and metamorphic rocks. Magnetite is black or brown-black with a metallic luster, has a Mohs hardness of 5-6, and leaves a black streak.

The IUPAC chemical name is iron oxide and the common chemical name is ferrous-ferric oxide.

In addition to igneous rocks, lodestone also occurs in sedimentary rocks, including banded iron formations, and in lacustrine and marine sediments such as detrital grains and magnetofossils. Magnetite nanoparticles are also thought to form in soils, where they are likely to rapidly oxidize maghemite.

Magnetite is sometimes found in large amounts in beach sand. Such black sands (mineral sands or iron sands) are found in various places, such as Lung Kwu Tan of Hong Kong, California of the United States, and the west coast of the North Island of New Zealand. Magnetite is carried to the beach through erosion rivers and is concentrated through the action of waves and currents. Huge deposits have been found in banded iron formations. These sedimentary rocks have been used to infer changes in the oxygen content of the Earth’s atmosphere.

Due to its high iron content, lodestone has long been an important iron ore. It is reduced in blast furnaces to cast iron or sponge iron for conversion to steel.

Magnetic acetate tape audio recording was developed in the 1930s. The German tape recorder used magnetite powder as the recording medium. After World War II, the 3M Company continued to work on the German design. In 1946, 3M researchers discovered that they could improve magnetite-based tape, which used cubic crystal powders, by replacing the magnetite with needle-shaped particles of gamma ferric oxide (γ-Fe2O3).

Magnetite is the catalyst for the industrial synthesis of ammonia.