Hackmanite

Hackmanite is a sulfur-rich variety of sodalite that exhibits tenebrescence (reversible photochromism). This means it changes color when exposed to UV light, then slowly fades back in visible light. The color change is caused by sulfur radical anions (S₂⁻) trapped within the sodalite crystal structure.

When UV light hits the crystal, it triggers electron transitions in the sulfur defects, producing a violet to pink color. In daylight, the process slowly reverses, and the crystal fades to white or pale lavender. This cycle can be repeated indefinitely without damaging the crystal.

Hackmanite forms in nepheline syenite and related alkaline igneous rocks, as well as in metamorphosed limestone (marble) contact zones. The best gem specimens from Afghanistan form in marble alongside lapis lazuli, ruby, and spinel, all products of the same geological event.

The definitive test: expose the specimen to shortwave UV light and watch it turn violet or pink. Remove from UV and watch it slowly fade back to white or pale lavender. This tenebrescence is unique to hackmanite and a few other rare minerals.

Distinguish from sodalite (blue, lacks tenebrescence), fluorite (different crystal system), and kunzite (different color-change mechanism). Hackmanite also fluoresces strongly orange under longwave UV, which helps distinguish it from similar-looking minerals.