Hackmanite is an exceptionally rare sulfur-rich variety of sodalite which exhibits tenebrescence (the ability of minerals to change color when exposed to sunlight). It was first discovered in Greenland in 1896 by L. C. Boergstroem, and was later named after Victor Axel Hackman (1866-1941), a Finnish geologist. However, it wasn't until recently (1991) that the first gem-quality hackmanite deposit was discovered in Quebec, Canada.

Hackmanite appears pale to deep violet when first mined, but once exposed to sunlight, the color quickly fades to grayish or greenish-white. When placed back into a dark place or when exposed to short wave ultraviolet light, the violet color slowly returns. The change in color can be seen within seconds of being exposed to sunlight, but the return of the original color can sometimes take up to a week. The tenebrescence effect can be repeated indefinitely, but is detroyed by heating.

The tenebrescence effect seen in hackmanite is the rarest of all gemstone optical phenomena. It is often confused with the 'color change' phenomenon, where materials can shift color under different types of light sources. Another term for tenebrescence is 'reversible photochromism'. A common example of reversible photochromism can be seen in everyday color-change eyeglass lenses. Color-change eyeglasses darken when exposed to sunlight and lighten up again when they are brought indoors. The effect is caused by a photochemical reaction to UV radiation and is known to occur in only a few rare materials.