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 (with impurities such as manganese, chromium and other metals) sometimes 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.

See: Wikipedia

Also see :Lightningplugin-autotooltip__plain plugin-autotooltip_bigLightning

High powered lightning discharges are happening somewhere on Earth around 100 times every second.

A possible mechanism for the very substantial electrical charges within the clouds was put forward in 1978. Following lab-based experiments, it was suggested that the charges arise from the static-electric interactions of graupel (slush) and ice crystals moving within the cloud (due to convection currents caused by widely differing air temperatures) .