(IDEX Online) - Seasoned diamond hunter Adam Hardin has found his biggest stone yet - a 2.38-carat brown gem.He's been a regular visitor to the Crater of Diamonds State Park, in Arkansas, for a decade and says he's found hundreds of smaller stones. But this is by far the biggest.It's the USA's only active diamond mine, where visitors pay $10 a day to sift the soil - and keep any gems they find.They wet-sift using a screen set to wash away soil and separate the gravel by size. "It was right in the middle when I flipped my screen over," Hardin said. "When I saw it, I said, 'Wow, that's a big diamond!'" Park Interpreter Waymon Cox said the diamond is about the size of a pinto bean, with a coffee brown color and a rounded shape."It has a metallic shine typical of all diamonds found at the park, with a few inclusions and crevices running all along the surface."So far this year 260 diamonds have been registered at the park, weighing a total of over 44 carats. Pic, courtesy Crater of Diamonds State Park, shows Adam Hardin and his diamond.