Cumberlandite, Iron Mine Hill

The state rock of Rhode Island comes from one four-acre lot, and it is faintly magnetic.

Cumberlandite is a melanocratic troctolite, sometimes called a melatroctolite, and it is found at Iron Mine Hill in Cumberland, Rhode Island, in a lot of about four acres. The US Geological Survey records Iron Mine Hill in Providence County as a prospect for iron and titanium, which is the rock stated as a commodity. Its high iron content is also why it is slightly ferrimagnetic.

It did not stay in its four acres. Glaciers carried it, and traces turn up scattered across the Narragansett Bay watershed as far as Martha's Vineyard and Block Island, sitting in glacial deposits that stretch south from the origin down to the southern shores of Narragansett. If you find a piece well away from Cumberland, ice put it there.

Colonial settlers worked out its value as ore during the 18th and 19th centuries, and the records show it was being smelted as early as 1703. It went into cannon: the metal was used in forging guns for the Siege of Louisbourg in 1745, and possibly during the American Revolutionary War as well.

It is easy to recognise once you know it. Cumberlandite weathers to a brownish black shot through with white crystals, carries secondary chlorite and saussurite, and is noticeably denser than the common granites and metamorphic rocks around it. Pick up a piece and the weight tells you before the colour does.

One correction to a common belief. Despite the name and the state designation, it is not unique to Rhode Island: cumberlandite is also found at Taberg in Sweden. The four-acre lot is still the classic locality, and it is small, so treat it as a place to look rather than a place to fill a bucket.