Some minerals will produce light when heated to a certain temperature.
Chlorophane, a variety of fluorite, exhibits one of the better known examples
of thermoluminsecence. In some cases, even the heat from your hand will
cause it to glow green. After several minutes of heating, the green light
will fade and the process cannot be repeated. The specimen above is from
Trumbull, CT. Chlorophane is very photo reactive and even a few minutes
of exposure to normal light, UV light, or heating will destroy it's
blue green fluorescence. Heating releases electrons that have been trapped
in a higher energy state for hundreds of thousands of years. Some minerals
are known to fluoresce
only when cooled.
Images © 2003 James O. Hamblen