Tenebrescence is an incredibly rare gem property that is also known as reversible photochromism. It describes the phenomena within a mineral or gemstone that will change colour when exposed to sunlight but will slowly turn back to its original colour when taken back into a darkened room.
It is different to the colour change phenomenon because that describes an instant change. If you move between daylight and candlelight with an Alexandrite, for example, the colour will immediately flick between green and red. With tenebrescence, the process is slower and will hold for a little while after the gem is moved between light sources. The time it takes and the depth of the colour will vary between gemstones.
It can be seen in the gemstone Hackmanite, a rare variety of Sodalite which was first discovered in Greenland in 1896. When mined the gem is a strong violet colour, but fades to a grey colour when exposed to sunlight or another ultraviolet light source. Interestingly, Hackmanite can also be found in Afghanistan and Myanmar (Burma) but with a colour shift in the opposite direction. Mined white or clear, they will shift towards a reddish-pink or violet colour in sunlight, as seen in our Burmese specimen in these pictures.
One of the most unusual things is that it’s an effect that doesn’t wear off, it’s believed to be repeatable indefinitely. Heat treating, however, will remove the tenebrescent properties forever. As well as Hackmanite, tenebrescence has been observed in Spodumene, some Scapolite varieties and Tugtupite. This photochromic process has been recreated synthetically and is used in the lenses of glasses that darken automatically in bright sunlight and lighten again when indoors.
View our limited range of UV Colour Change Hackmanite on our Gem Collector website here.
MORE GEMSTONE PHENOMENA
Fluorescence and Phosphorescence