Hackmanite

Hackmanite is a variety of sodalite known for its intense tenebrescence. Tenebrescence is a rare optical phenomenon, sometimes called reversible photochromism, in which a mineral changes color when exposed to ultraviolet (UV) light and then gradually returns to its original color in darkness or under visible light.

The physical mechanism involves UV radiation exciting electrons within the crystal lattice. These electrons become trapped at defects known as F-centers (color centers). The trapped electrons absorb specific wavelengths of visible light, producing a color change—typically from colorless to pink or purple in hackmanite. Exposure to white light or heat provides enough energy to release the trapped electrons, restoring the mineral to its original state.

The return to the original color may occur within minutes or may take several days, depending on the sample and environmental conditions.

This specimen is from Badakhshan, Afghanistan and is 81 X 57 X 55 mm (3.2 x 2.25 x 2.2 in) and 249 g (8.8 oz) and was purchased October 2025 for $50 from Shafiqullah Alamyar of Alamyar Fluorescent Minerals.  Photographs here are by Shafiqullah Alamyar from the ad with the exception of the two for scale.

For scale.  Note I only exposed the area to the right to UV so the crystals to the left and bottom are the normal color and the one on the right is showing the color change.