Hackmanite

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Hackmanite
Chemical composition Natrium aluminium silicate
Crystal system Cubic
Habit Aggregate
Hardness 5 - 6
Refractive index ± 1.48
Specific gravity 2.15 - 2.40
Tenebrescent Hackmanite before exposure to sunlight
Photo courtesty of The Gem Trader

Tenebrescent Hackmanite after exposure to sunlight
Photo courtesty of The Gem Trader

Hackmanite is the name given to natural Sodalite which has unique properties of fluorescence. While Hackmanite from Afghanistan and Burma are the same mineral, different ions, including but not limited to Sulfur, cause the stones to react differently.
Afghan Hackmanite, while not common, can be clean enough to facet. Burmese Hackmanite is translucent at best, but is often opaque.
Afghan Hackmanite is colorless to very light pink or violet in its natural state. Burmese Hackmanite can be light to medium gray, light blue, light pink or light violet.
When subjected to SW UV lighting, Afghan Hackmanite will darken to light to medium violet over a period of 3-5 minutes. Burmese Hackmanite, however, will darken to intense dark violet in a matter of seconds.
Sunlight has no effect on most Afghan Hackmanite. Burmese Hackmanite, however, will gradually darken before your eyes over a period of 3-5 minutes when exposed to sunlight.
Afghan Hackmanite will fade to its original lighter tone in a matter of seconds or minutes. Burmese Hackmanite, however, once 'charged' with sunlight or SW UV, will gradually and very slowly fade over a period of 1-7 days.

Click here for GemologyOnline's Hackmanite Article