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Fluorite
This angular sample is a composition of fluorite (purple) mixed in with siderite, possibly from the Weardale region of Northern England.
Fluorite is the mineral form of calcium fluoride, veins of which often occur running through metallic minerals. Siderite in turn is a widespread iron carbonate. Pure fluorite is colourless, but the addition of impurities mean fluorite crystals rank amongst those with the widest colour range of all minerals. Perhaps the most significant feature of fluorite was first discussed by physicist George G. Stokes in 1852, upon observing its glowing luminescence when lit with ultraviolet light. He named this phenomenon ‘fluorescence’, not only coining the term but describing the process in an enduring rule known as Stokes Shift.