The lodestone, a piece of magnetite, which is naturally magnetic, was vital in the early centuries of navigation to ensure that the ship's compass worked properly. Until the mid-18th century, when improved compasses were developed, compass needles lost their magnetism quite quickly and had to be re-magnetised by stroking a lodestone along the needle's length.

Lodestones were often mounted in frames of brass, bronze or silver, and sometimes had an iron or steel keeper - a bar to help preserve their magnetic power. This lodestone has a brass frame with a carrying loop on top, and is 'armed' with two pieces of steel at the bottom, which help to increase the magnetic strength of the lodestone. This is a particularly large lodestone, which weighs over 9Kg (20 lbs) and was probably used for scientific experiments rather than for navigation.