Dartmoor is an area of raised moorland formed by the intrusion of granite, an igneous rock from the the earth's mantle. The semi-molten rock forced its way through the surrounding country rock; later mineral fluids emanating from the mantle flowed through the fissures in the granite. These then cooled and crystallised forming mineral veins termed lodes. The lodes are usually vertical or near vertical.

The lodes run east to west along the flank of the Moor in the triangle bounded by Bovey Tracey, Moretonhampstead and Hennock. In this part of the Dartmoor environs they contain iron oxide in the form of micaceous haematite, Fe2O3, with smaller quantities of red oxide, iron sulphide or Pyrite (Fool's Gold) and tin oxide or Cassiterite (tin ore). The lodes also contain gangue (unwanted) minerals such as quartz. The micaceous haematite, locally known as 'Shiny Ore' due to its splendent lustre, occurs in the form of minute, greyish-purple mica-like flakes.. The material is also referred to as micaceous iron oxide (MIO).

These deposits were successfully worked by a number of mines, the largest of which was the Great Rock Mine at Hennock. Great Rock was run during the twentieth century by the Ferrubron Manufacturing Company, who also acquired Kelly and several other mines in the area at the same tirne. All except Great Rock and Kelly Mine were immediately closed, presumably to ensure a monopoly on the production. Virtually all the ore mined was used to manufacture anti-corrosion paint (MIO paint). Later, when British Standards were imposed on the paints used by the Royal Navy and the railways, the material produced by Ferrubron was found to reach the required specification without further processing.

In 1951 Kelly Mine closed followed in 1969 by Great Rock Mine, bringing the production of 'Shiny Ore' in Britain to an end.