Franklin is a borough in Sussex County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 5,045 reflecting a decline of 115 (-2.2%) from the 5,160 counted in the 2000 Census, which had in turn increased by 183 (+3.7%) from the 4,977 counted in the 1990 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. Settled in the 17th century, the village known as Franklin Furnace after Benjamin Franklin, developed near iron mines and iron smelting operations located along the Wallkill River. In the early 19th century, zinc deposits in the area began to be developed commercially. For most of the century many small companies mined zinc and iron in the Franklin area. In 1897 all zinc mining efforts merged into the New Jersey Zinc Company, which was a major controlling factor in the development of Franklin. Immigrants from Russia, Britain, Hungary and Poland joined the work force at the mine. The population, 500 in 1897, had swelled to 3,000 by 1913. On March 18, 1913, the Borough of Franklin was incorporated from portions of Hardyston Township, based on the results of a referendum held on April 23, 1913.
According to the United States Census Bureau, the borough had a total area of 4.43 square miles (11.47 km2), including 4.36 square miles (11.28 km2) of land and 0.07 square miles (0.19 km2) of water (1.60%).
The borough borders the boroughs of Hamburg and Ogdensburg, as well as Sparta and Hardyston townships.
Franklin Furnace provides many examples of the complex mineralogy of the area.
The 2010 United States Census counted 5,045 people, 1,936 households, and 1,316 families in the borough. The population density was 1,121.6 inhabitants per square mile (433.1/km2).