Hackmanite
Last updated: April 2026
Hackmanite is the tenebrescent variety of sodalite, a sodium aluminum silicate chloride mineral. Tenebrescence (also called reversible photochromism) is the defining property: the stone becomes purple-pink when exposed to shortwave UV radiation and reverts to colorless or pale when exposed to bright visible light or heat. Major sources include Afghanistan (classic purple tenebrescence), Myanmar, Greenland (Ilímaussaq complex), and Canada (Mont Saint-Hilaire). The phenomenon is caused by sulfur-related color centers (S₂⁻ radical anions) that form and break under UV/visible light cycling. Gem-quality faceted hackmanite is rare; most material is cut as cabochons or used ornamentally.
Hackmanite has a specific gravity of 2.27 and a refractive index of 1.483–1.487, with a Mohs hardness of 5.5–6.0. It is singly refractive.
Physical & Optical Properties
Measured with a polariscope, a refractometer, a hydrostatic balance, a Chelsea filter, a UV torch, a loupe, a darkfield loupe and a spectroscope — step-by-step for each.
Through the refractometer
Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.
Related: Sodalite Varieties
Key Differentiators
- Tenebrescence — reversible color change under UV: colorless/pale in daylight, purple-pink after SW UV exposure
- Color reverts to pale after exposure to strong visible light (incandescent or sunlight)
- Strong orange-pink LW UV fluorescence — same as sodalite group
- RI 1.483–1.487 and SG ~2.27 identical to sodalite — tenebrescence is the distinguishing feature
- Isotropic (cubic) — singly refractive; no birefringence
Common Simulants
- Amethyst: Purple quartz; doubly refractive. RI 1.544–1.553 (much higher than hackmanite ~1.483–1.487); SG 2.65 (heavier than hackmanite ~2.27). Amethyst shows no tenebrescence (color change under UV). Use refractometer to confirm.
- Fluorite: Purple fluorite is isotropic; shows perfect octahedral cleavage in four directions. RI ~1.434 (lower than hackmanite ~1.483); SG ~3.18 (heavier). No tenebrescence; strong UV fluorescence (often bright blue-violet).
- Sodalite: Hackmanite is a variety of sodalite — may be sold simply as 'sodalite.' Non-tenebrescent sodalite lacks the reversible color change under UV/visible light that defines hackmanite. Blue-violet body color without fading is typical non-tenebrescent sodalite.
Measurement Guides
Identifying a hackmanite? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
Try Loupewise Free →Studying for the GIA, FGA, or FEEG practical? Hackmanite is one of the 132 species in Loupewise’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.
Practice exams →