lodestones

Lodestone is a naturally magnetized piece of the mineral magnetite (Fe3O4). It is one of the few naturally occurring materials that can retain a magnetic field at ordinary temperatures, and it has long been prized as a source of permanent magnetism. The name lodestone comes from the Old English loadstone, meaning “leading stone,” reflecting its historic use to guide travelers and navigators.

Most lodestones form when magnetite crystals acquire a permanent magnetic moment through exposure to Earth's magnetic field, a process known as thermoremanent magnetization as rocks cool below the Curie temperature, or by later geological processes that align magnetic domains. Natural lodestones are found in various rocks and in placer deposits, and historically the best-known sources were near Magnesia in ancient Greece, from which the term magnet originates.

Magnetite is ferrimagnetic; lodestone exhibits a stable, intrinsic magnetic dipole and is attracted to iron. It has a Curie temperature of about 585°C, above which magnetization is lost. Lodestones can be demagnetized by heating, strong shocks, or exposure to intense magnetic fields; they can be remagnetized by exposure to another magnetic field.

Historically, lodestones were used to make early magnetic compasses, with needles magnetized by rubbing or by contact with magnetized stones. By medieval times, magnetized needles and compasses made navigation possible for ships and caravans, and the term magnet derives from Magnesia. Today, lodestone is mainly of historical and mineralogical interest, with magnetite remaining the primary source of natural permanent magnetism in geology.