Lodestone is a naturally magnetised form of the mineral Magnetite - which is mostly composed of Iron(II,III)oxide (Fe3O4).
It's of great historical importance in that it led to the discovery and study of magnetism in general, and also made possible the invention of the magnetic compass, which made trans-oceanic voyages feasible.
Magnetite is not usually permanently magnetisable, although certain crystalline versions of it (sometimes with impurities such as manganese, chromium and other metals) are. But the question remains 'How did the lodestone become permanently magnetised?'
The Earth's intrinsic geomagnetic field is far too weak to be able to induce the permanent magnetic properties of lodestone.
A theory that lightning strikes could magnetise the stones has been experimentally tested (see Geophysical Research Letters 26(15):2275-2278) but found that a stone would have to be 5 - 10 cm away from a direct strike to become magnetised.
Other possibilities include large-scale electromagnetic pulses from extreme solar flares, or from asteroid impacts.
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