Video of Tenebrescent Sodalite (Hackmanite) and Tugtupite
Tenebrescence is when minerals change color when exposed to sunlight, also known as reversible photochromism.
Sodalite from Greenland (hackmanite) will darken in color when exposed to shortwave ultraviolet light (or sometimes just sunlight). When exposed to this UV they fluoresce brightly. Once the UV is removed they have turned a darker color, some a deep purple. When exposed to a bright white light they will fade back to their original color rather quickly.
Tugtupite experiences this same phenomenon. UV causes it to turn a beautiful red color - and makes it a valuable gemstone. It does not fade as rapidly as sodalite, sometimes not fading at all or very little.
The video below shows this process. The three pieces on the right are tugtupite (reddish colors) and the three on the right are sodalite (they will all have a grape/purple color when tenebrescent).