Quick answer: Franklin and Sterling Hill in Sussex County, New Jersey, are zinc mines famous for the world’s most spectacular fluorescent minerals. Under shortwave UV light, willemite glows bright green and calcite glows red-orange. The deposits have produced more than 350 mineral species, many of them first discovered there, which makes Franklin one of the richest mineral localities on Earth.
Under ordinary light, a typical Franklin rock looks unremarkable: greyish white with some brown and black grains. Switch off the lights and turn on a shortwave ultraviolet lamp, and it blazes vivid green and fiery red. That display made two small mining towns in northwest New Jersey famous among collectors worldwide.
Two mines, one strange ore body
Franklin and Ogdensburg sit a few miles apart in Sussex County. Each had a large zinc deposit, the Franklin Mine and the Sterling Hill Mine, set in ancient marble. The ore was not the usual zinc sulfide that most of the world mines. It was a rare combination of oxide and silicate minerals: franklinite (black), zincite (red to orange), and willemite (often green), along with calcite.
How the deposits formed is still debated, but they are very old, well over a billion years, and were later metamorphosed deep in the crust. That long, complicated history, with zinc, manganese, iron, and dozens of other elements cooked together, is why so many unusual minerals grew there.
A world record for variety
More than 350 mineral species have been identified from the Franklin and Sterling Hill area, one of the highest counts for any locality on Earth. Dozens were first discovered there, and a number are still known from nowhere else. Franklinite itself is named after the town.
The fluorescent capital
The glow comes from specific atoms in the crystal structure. In willemite, small amounts of manganese act as activators that absorb invisible ultraviolet light and re-emit it as green. In the calcite, manganese produces red-orange. Other species add blue, violet, and yellow. We explain the physics in fluorescence, and in phosphorescence we cover minerals that keep glowing after the lamp goes off, which some Franklin material does.
New Jersey officially recognized Franklin as the “Fluorescent Mineral Capital of the World,” and the material is the benchmark every other fluorescent locality is compared to.
After the mining ended
The Franklin Mine closed in 1954. Sterling Hill kept operating until 1986 and was the last working underground mine in New Jersey. Both survive as places to learn about the deposits. The Sterling Hill Mining Museum runs underground tours, and the Franklin Mineral Museum has a well-known fluorescent display and has offered supervised collecting on old mine dumps.
How Franklin fits the bigger picture
Zinc usually shows up as sphalerite, a sulfide, as we describe in sulfide minerals. When sulfide ores weather, zinc can end up as the carbonate smithsonite, covered in smithsonite and Laurion. Franklin is the rare case where zinc crystallized as oxides and silicates instead, and that is what makes it so distinctive.
New Jersey has another world-class mineral story, told in Paterson’s trap rock quarries and the Watchung Mountains, but Franklin is its own world.
