New Jersey comprises five geologic regions, three of them shared with Pennsylvania. All of them have notable mineral deposits, and four of them have produced gemstones. Only a few of the known sites are publicly accessible, however. Near some old mining operations, "dumps" of rock with too-small quantities of useful minerals for industrial extraction are open to visitors.
New Jersey has its own native brand of quartz, known as "Cape May diamonds." They are unusually clear quartz pebbles, best found on Sunset Beach, to the West of the town of Cape May after an arduous trip down the Delaware River valley that has tumbled them smooth. Quartz pebbles and multicolored quartzite are also to be found almost at the other end of New Jersey's shore of Delaware Bay, in the Salem Jasper Gravel, in the shadow of a nuclear power plant. Off a student parking lot at Montclair State University, on the site of a former quarry, you may still find smoky quartz, amethyst and other stones including opals, a chemically similar, though sedimentary, stone that contains water.
Agates can be found at some of the same sites as quartz pebbles, and so can jaspers. South of Interstate Highway 78 in Somerset County is a site sometimes referred to as "Carnelian Brook." Malachite and serpentine can be found, along with other minerals, at an overgrown quarry in a Morris County park near Lake Valhalla. The nearby Watchung Mountains hosted some very early copper mines, and this area also offers finds of copper-containing gemstones such as malachite.
Iron was a major mineral in New Jersey's development from the earliest colonial days, and the gemstone forms of hematite, marcasite and pyrite are still to be found in several old and current quarries and mines. They were once found in large nodules at Cliffwood Beach on Raritan Bay, though development has made them harder to spot in the dark clay.
Though not generally recognized as a gemstone, New Jersey's true native mineral, found in the zinc mines in the state's northwest corner, is well known to rockhounds. The Sterling Hill Mining Museum in Ogdensburg offers several digging opportunities, both in the remains of the original mine's output and in dumps created for visitors' enjoyment.The Franklin Mineral Museum, in nearby Franklin, has three collecting areas, including the original 3.5-acre Buckwheat Dump, where new minerals have been identified in the 21st century. Franklinite contains both zinc and iron and has been described as resembling chocolate chips. The Franklin-area minerals are especially known for their fluorescence, best viewed under black light.
The Lime Crest Quarry, still operating outside of Sparta, has on occasion opened to the public for gem hunting under the auspices of the Franklin-Ogdensburg Mineralogical Society. Along with gems and minerals found elsewhere in the state, rock hunters have gone home from Lime Crest with blue-green apatite — until recently little known as a gemstone and also found at the Sterling Hill Mine — spinels, dravite (a form of tourmaline) and even rubies.