Different magnetic materials can be found in the crust of the earth. However, just lodestones have the ability to behave like a magnet. The fact that lodestones were the base of the first compasses, is still hidden in their name “lode”(=course)-stone. In 1994, electron microscopy revealed the secret structure of lodestones. It was known before that lodestones are made of oxidized magnetide (Fe3O4), but not all oxidized magnetide was lodestone. The secret of lodestones lay on an intergrowth of magnetite (Fe3O4) and maghemite (Fe2O3). Nevertheless, this structure does not explain the origin of the charge. In 1999, Peter Wasilewski and Günther Kletetschka dealed with the question how lodestones gets charged. They calculated the magnetic field which would be needed to charge a lodestone and came up with an interesting hypothesis: Lodestones are charged up by lightning strikes. Experiments with “controlled” bolts showed that the magnetic field created by bolts can increase the remanent magnetization (= magnetization which remains) in lodestones. That would also explain why lodestones are found “near the surface and not in deep mines”. "Lodestone: Natures only permanent magnet ‐ What it is and how it gets charged."
Peter Wasilewski and Günther Kletetschka Geophysical research letters 26.15 (1999): 2275-2278.
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