The Diamond in New Castle, Pennsylvania, is a historic public square located at the intersection of Washington and Jefferson Streets in downtown New Castle. It was part of the original town plat laid out in 1798 by surveyor John Carlysle Stewart, who designed the town around this open market space measuring approximately 440 by 190 feet. The Diamond served as the central hub and heart of the original settlement and remains a symbolic and geographic focal point of the city to this day.