The state rock of Rhode Island is on display at the Beavertail Aquarium for the summer.
The pieces of cumberlandite were discovered by aquarium intern Cameron Chadronet at Beavertail State Park. Cumberlandite is a magnetic, igneous rock that is only found in Rhode Island, and was designated as the state rock in 1966. The rock usually is black in color with green or white crystals.
Chadronet said he found the cumberlandite in eroded soil at Beavertail in May. He had been looking for a piece of the rock, and searched for about 12 hours during the course of a week. He found three samples by tapping rocks that looked like cumberlandite with magnets.
“The glacier pushed some down here, which is why we can find some in the bounds of the park,” he said. “I saw it, I recognized it and I used the magnet to prove that it was cumberlandite. I brought it here so we could show tourists what cumberlandite looks like.”
Aside from its magnetic properties, Chadronet said he knew the rocks were cumberlandite because they had a glossy luster and they had weathered from black to a browner shade. Two of the pieces that Chadronet found are small, but he said the third is much larger, weighing in at around 50 pounds.
Pieces of cumberlandite can be found throughout Rhode Island. Bedrocks of the state rock, however, are rare.
Mark Dennen, a supervising environmental scientist for the Rhode Island Department of Environmental Management, said the only large outcropping of the rock in the world is found in a four-acre area in Cumberland.
“When you find pieces of it in other parts of the state, you know it came from northern Cumberland,” Dennen said.
Dennen said it is unknown why cumberlandite is only found in abundance in that small area. One of the aspects that makes rock distinctive is the quantity of iron, which is what makes it magnetic.
“It’s rare in its composition and it’s rare in its texture,” Dennen said. “It has very high levels of iron. So high, that sometimes it turns reddish because it rusts. It has so much iron that it will move your compass.”
In addition to its high amount of iron, Dennen said cumberlandite also has magnesium and titanium in unusually high levels for a rock. It is also heavy compared to other rocks; Dennen compared it to a lead weight. Cumberlandite is also porphyritic, which means it went through two stages of cooling, and some of the crystals are large while others are microscopic.
Although cumberlandite is only found in significant quantities in Cumberland, Dennen said fragments — from pebbles to boulders — can be found scattered because they were moved by glaciers during the Ice Age.
“Normally you don’t know exactly where it came from because most kinds of rock would be over a very extensive area,” Dennen said. “If you find a rock anywhere that has cumberlandite in it, you know it came from that site in northern Cumberland because it’s the only place that it occurs.”
Dennen said that locations where pieces of cumberlandite have been found outside of Cumberland, including Jamestown and Block Island, reveal the trajectory of the glacier that moved through Rhode Island.
“The glacier picked it up in that one specific location and it dropped it in Jamestown,” he said. “We can trace the movement of a glacier because that rock is so unique. The cumberlandite has helped us understand a lot about the last Ice Age with that train of pebbles.”
The cumberlandite that Chadronet found was on display at the aquarium June 1 when Gov. Dan McKee visited the complex. McKee was born in Cumberland and was the town’s mayor from 2007-15. Chadronet said the governor enjoyed the visit to the aquarium.
“He seemed to strongly approve of it, especially since it was free to the public,” he said. “After I showed him a lot of the living creatures, I moved on to the non-living specimens and I brought up the cumberlandite. He knew about cumberlandite and I was able to tell him some stuff he found interesting.”
Chadronet, for example, said there are stories of Colonists who made unsuccessful attempts to make cannons out of cumberlandite during the Revolutionary War. The cumberlandite, however, cracked apart while those cannons fired because the rock was too soft.