The Bon Ami Mine's deposits of the mineral hyalite glow fluorescent green under ultraviolet light
It's 8 p.m. on a windy autumn night, and I'm walking down a dirt pathway to the entrance of the Bon Ami Mine in Little Switzerland, North Carolina, located about 50 miles northeast of Asheville. Decades ago, the mine was fully operational and prized for its abundance of gems and minerals like feldspar, mica and quartz, but today it's part of Emerald Village, a popular mining attraction for rock hounds and anyone who's interested in learning more about the Blue Ridge Mountains' mining history. While Emerald Village is chock full of activities for all ages, including panning for riches like emeralds and garnets, and exploring its rambling, multi-story mining museum that contains mineral specimens found onsite, I'm here on this seasonally chilly Saturday night to experience its Black Light Mine Tour. The exclusive event is held only ten times a year and is the best way to see the mine's towering walls and ceiling, which are encrusted with large deposits of hyalite, a form of opal known for its glass-like transparency. When viewed under ultraviolet light, the normally colorless gemstone glows a brilliant fluorescent green. Hyalite's intoxicating glow, which immediately calls to mind kryptonite from the Superman comic book series, is due to the gemstone's trace amounts of uranium, a chemical element that glows when exposed to ultraviolet light. Uranium was once a popular material in glassmaking simply because it fluoresces under UV light. "The UV excites the electrons above the ground state and gives off photons as the electrons transition back to the ground state," an article in Collectors Weekly explains. "The fluorescence is just an inherent property of the uranyl compound in the glass." "When you look at hyalite in the daylight, it's colorless and often has a bubbly appearance," says Jeff Post, a curator of the mineral and gem collection at Smithsonian's National Museum of Natural History and a research mineralogist. "It's a clear layer that can be found on the…