Kunzite, the transparent gemstone variety of the mineral Spodumene (Lithium Aluminum Silicate, LiAl(SiO3)2, an Inosilicate) typically displays a colorless to light pink hue. This coloration is attributed in literature to a Manganese impurity. Samples of Kunzite happened to be one of the many minerals and gemstones that I irradiated. It has been described in literature as having a short-lived green coloration upon natural irradiation (viewed in freshly mined specimens straight out of the ground) and after irradiation with ionizing radiation. As it turned out, the green transformation was quite spectacular. The gamma induced changes were accomplished used 2-5 megarad dose from a 1000 rad/min 662 KeV Cs-137 gamma irradiator.
The transformation is only more spectacular once the lights were turned off. Viewing samples of irradiated Calcite in a darkened room to observe another interesting effect, room temperature thermoluminescence, I noticed a sample of irradiated Kunzite also glowing with a similar (although not identical) orange-red emission. While initially I deduced this to be a similar low temperature thermoluminescence, the effect was then showed to be a delayed fluorescence (otherwise known as phosphorescence). Shorter wavelengths tend to charge phosphorescent samples best, and these irradiated Kunzite samples can be significantly charged by a UV flashlight.
Phosphorescence is a quantum mechanical effect caused by so-called “forbidden transitions,” an atomic analog of the nuclear effect of metastable isomers described elsewhere on this site. Simply, whereas fluorescence is the prompt (microsecond timescales) reemission of incident energy, phosphorescence occurs via forbidden energy state transitions that occur with very low probability thus delaying the reemission over much longer timescales.
Typical phosphorescent materials (the kind you will find in toys, safety lighting, etc.) are Zinc Sulfide (older, typically doped with Silver or Copper) and Strontium Aluminate (newer, brighter, Europium doped), and while many minerals demonstrate a prompt fluorescence under UV illumination, significant delayed emission is exceedingly rare (occasionally seen in Zinc Sulfide and Silicate minerals as well as certain varieties of Calcite), and the phosphorescence of the irradiated Kunzite is by far the most significant I have witnessed from a natural mineral. This represents the only case of induced phosphoresce I have been able to uncover.
The study of phosphorescence is directly tied to the discovery of radioactivity. Indeed the eminent turn of the century physicist Sir William Crookes was drawn into the study of the nucleus by this effect, and was supposedly fascinated by the effects of ionizing radiation on gemstones (Diamonds and Kunzite in particular), and many colored diamonds (green and yellow) are mildly radioactive from being submerged in a Radium Bromide powder (and later Americium-241 Oxide) using a method developed by Crookes.
A few more additional pieces of experimental evidence I’ve collected allow us some more insight to the specific cause of this unique effect. Like other dielectric materials, the stored energy can be released by heating, causing the crystals to revert to their original state (pink-clear with no fluorescence or phosphorescence), and this cycle can be accomplished as needed. Most telling perhaps was the below sample of Kunzite. As opposed to a homogenous emission, it is heterogeneously distributed across a gradient and in a specific corner of the specimen. This would suggest the effect to be impurity driven.