Waco Gem and Mineral Club, Waco, Tx

18.6 Special Effects of Some Fluorescent Minerals


18.6: Special Effects of Some Fluorescent Minerals

In addition to changing color and glowing as if lit from within, some fluorescent minerals exhibit still more special effects. These include phosphorescence and tenebrescence.


Kids today might not have experienced this, but folks from my generation will recall turning off the television in a darkened room late at night and watching the screen slowly fade to black. Some fluorescent minerals do the same thing, momentarily holding a faint glow after a fluorescent lamp has been switched off, in a phenomenon known as phosphorescence. It・s especially noticeable in pink calcite rhombohedrals from Nuevo Leon, Mexico, often sold at gem and mineral shows. A large chunk makes a great addition to Activity 17.8, The Amazing Mineral Magic Show! It is truly neat to watch as the calcite continues glowing in the dark after fluorescent lights are switched off, with the glow gradually fading away. Phosphorescence, or 'afterglow', is also observed in scapolite, some celestine and barite, gypsum, hydrozincite, Terlingua calcite from Texas, and other minerals.


I learned about tenebrescence at one of those gem shows that take place in the rooms of a big hotel. My wife and I entered Room 204, whereupon an elderly man from Afghanistan waved a pale purple crystal in our faces and urged us to join him in the bathroom. Our hesitation when he closed the door turned to alarm when he switched off the lights to total darkness. Then, with a 'click', we saw his bright smile illuminated by a fluorescent lamp and the crystal glowing bright apricot orange. I was ready to buy it then and there, but the show wasn・t over. When we emerged back into the light of the hotel room, I saw that the formerly pale crystal was now a vivid raspberry color! The man・s smile grew even larger as I reached for my wallet. Later at home, we saw that the color of our wonderful new acquisition had reverted back to pale purple when we left it exposed to daylight. Thus, my first-hand lesson about tenebrescence, a property by which a mineral can change color when exposed to UV light (particularly SW UV), then fade under daylight, only to regain its brighter colors with a little more UV exposure. This color reversal can be repeated indefinitely. The property of reversing color with changes in light radiation has been called reversible photosensitivity or reversible photochromism, or more commonly, tenebrescence. Hackmanite (a variety of sodalite) is the mineral perhaps most well known for exhibiting this special effect. You can show the same effect with 'photosensitive eyeglasses' that self-darken into sunglasses on exposure to strong sunlight and then turn clear again indoors. Other minerals that can exhibit tenebrescence include tugtupite, spodumene, 'chameleon diamonds', and some barites.