The magnetic susceptibility (\(\kappa\)) defines how easily a rock becomes magnetized when it is subjected to a magnetic field \(\mathbf{H}\). The relationship is:

\[\mathbf{M}=\kappa \mathbf{H}.\]

Background information about susceptility, how it is measured and tables of values are provided in magnetic susceptibility. In a geological context, magnetite-rich rocks are the main source of magnetic signal, both in terms of abundance and susceptibility. In engineering, buried metallic objects are generally highly susceptible, but they can also carry a substantial remanent component. The image below shows a rock that has a remanent component that it is strong enough for the rock to act as a magnetic and attract some paper clips, just as a bar magnet does.

Magnetite Lodestone.jpg
By Ryan Somma - Magnetite Lodestone, CC BY-SA 2.0,