Fluorite

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Choice cluster of lustrous green gemmy fluorite crystals without matrix. Brightly fluorescent. Many of the crystals exhibit twinning. The Weardale district of County Durham, England, situated within the Northern Pennine Orefield, has a mining history dating back to the 12th century. While early operations targeted lead and iron ores, the discovery of the fluxing properties of fluorite (fluorspar) in steelmaking during the late 19th century transformed the local mining industry. Geologically, the mineralization is hosted predominantly within Carboniferous sedimentary rocks, specifically the Great Limestone member. Hydrothermal fluids, focused by the underlying Devonian-aged Weardale Granite, deposited fluorite, galena, and quartz within fault-bounded veins and horizontal replacement deposits known as "flats". Commercial fluorite mining peaked in the 20th century before ending with the closure of the Frazer's Hush mine in 1999. Today, the district remains internationally recognized for producing sharply formed fluorite crystals, often exhibiting penetration twinning and a distinctive daylight fluorescence triggered by trace rare-earth elements within the crystal structure.