Franklin is a borough in Sussex County, in the U.S. state of New Jersey. As of the 2020 United States census, the borough's population was 4,912, a decrease of 133 from the 2010 census count of 5,045, which in turn reflected a decline of 115 from the 5,160 counted in the 2000 census. Franklin, known as the "Fluorescent Mineral Capital of the World," is located over a rich ore body containing more than 150 minerals, many of them fluorescent and 25 of which are found nowhere else on earth.