One of the weirdest properties in the gemstone repertoire is something called tenebrescence or reversible photochromism. That’s when the color changes from exposure to light, like self-adjusting sunglasses, and then back again after a period in darkness. Several other stones will do this but
, especially some of the varieties from Afghanistan and Burma, carries this effect to extremes as you can see in picture 4: deep purple by day, pale blue by night.
A member of the sodalite or “Princess Blue” group, hackmanite distinguishes itself by the additional presence of sulfur in its makeup. Though discovered in Greenland way back in 1896 it’s still relatively unknown. It’s too soft for a ring or any other application that may take much of a beating, but fine for something like a pendant. It glows orange to red under UV.