Cumberlandite

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Cumberlandite is Rhode Island's state mineral. It is only found in large concentrations on a four-acre lot in Blackstone Valley, Cumberland, RI, and in traces scattered throughout the Narragansett Bay watershed. Due to its high amounts of iron, it is slightly magnetic.

The unique mineral was formed over 1.5 billion years ago by volcanic activity. The rocks were discovered by settlers hundreds of years ago, and were initially deemed valuable for the manufacture of cannons and farm tools in the 18th and 19th centuries. The mineral also contains a relatively high level of titanium, also valuable for the manufacture of tools. Colonists tried to make cannons from the mineral during the Revolutionary War, but the cannons cracked because of the weakness of the iron.