Hackmanite: The Gemstone That Changes Colors

Hackmanite is a rare variety of sodalite, a mineral belonging to the feldspathoids group. It is known for its unique color-changing property called tenebrescence, which allows it to temporarily change color when exposed to different light sources.

Hackmanite tenebrescence is a fascinating phenomenon that makes this gemstone truly unique. Tenebrescence is the ability of certain minerals to change color after exposure to ultraviolet (UV) light. In the case of hackmanite, the stone will typically darken to a deeper purple or violet shade after being under a UV light source. This color change is temporary, and the stone will gradually return to its original color over time when placed back in darkness.

That’s when the color changes from exposure to light, like self-adjusting sunglasses. Several other stones will do this but hackmanite, especially some of the varieties from afghanistan and burma, carries this effect to extremes as you can see pale blue to deep purple.

Hackmanite color change
This Gemstone Switches Colours in an Instant. Hackmanite  from the Koksha Valley, Afghanistan

The exact mechanism behind hackmanite tenebrescence is not fully understood, but it is believed to be related to the presence of certain impurities in the stone's crystal structure. These impurities can absorb UV light, which excites electrons within the mineral and causes them to move to higher energy levels. When these electrons return to their ground state, they release energy in the form of visible light, which gives the hackmanite its deeper color.

Tenebrescence refers to a reversible color change in which certain minerals darken or change color in response to radiation of one wavelength and lighten or change back to their original color upon exposure to a different wavelength. These changes usually take effect with exposure to Shortwave UV, Longwave UV and sunlight.

This effect can be repeated indefinitely, but is destroyed by heating. Tenebrescent minerals include the Hackmanite variety of Sodalite, some Scapolites and Tugtupite.

White Hackmanite, from a few sources, may turn raspberry red after exposure to shortwave UV light and the color fades rapidly in sunlight. A recent find of colorless Scapolite from Badakhshan, Afghanistan changes to a saturated, medium blue when exposed to shortwave UV light.

When the shortwave UV light is removed the stones immediately begin to fade and return to colorless within a few minutes. Tugtupite will darken in color from from light pink to deep rose red when exposed to shortwave UV light and fade back to light pink when removed from the UV light.

The rate at which hackmanite darkens and lightens can vary depending on the specific composition of the stone. Some hackmanites may only take a few minutes to reach their full color change, while others may take several hours. The length of time the stone retains its darkened color will also vary, with some stones returning to their original state within a few hours and others taking weeks or even months.

In Kunzite, the pink variety of Spodumene, the pink color may fade with prolonged exposure to light. This has earned Kunzite the nickname of the "evening stone" since wearing it during daylight hours is not recommended. Maybe these minerals should be termed "irreversible" photochromic minerals - but that would be an oxymoron since "photochromic is defined as "reversible". 

Hackmanite's properties

  • Composition: Sodium aluminum silicate with sulfur and chlorine (Na₈Al₆Si₆O₂₄(Cl₂,S)) - A variety of sodalite.
  • Color: Pink, violet, lilac, gray, greenish-white, yellow. Exhibits tenebrescence, changing to a deeper purple or violet under UV light and fading to grayish-white or greenish in sunlight.
  • Transparency: Transparent to translucent.
  • Luster: Vitreous (glassy) or greasy.
  • Crystal System: Isometric (cubic).
  • Streak: White.
  • Hardness: 5.5 to 6 on Mohs scale.
  • Cleavage: Uneven or conchoidal (breaks with a smooth, curved surface).
  • Fracture: Poor, one direction.
  • Habits and Forms: Usually found in massive (shapeless) aggregates, sometimes cubic or octahedral crystals.
  • Specific Gravity: Same as density (2.14 - 2.40)
  • Fluorescence: Strong to moderate orange or yellowish orange under long-wave UV light. Can be bluish white or orange under short-wave UV light.
  • Pleochroism: Not noticeable.
  • Refractive Index: 1.479 - 1.487
  • Inclusions: Often intergrown with other minerals, especially non-tenebrescent sodalite.


Hackmanite: The Gemstone That Changes Colors
Hackmanite Raw Specimen

Hackmanite Uses

Hackmanite is a variety of the mineral sodalite and is known for its unique ability to exhibit tenebrescence. Here are some potential uses and aspects related to hackmanite:

Jewelry: Hackmanite's color-changing property makes it an interesting and sought-after gemstone for jewelry. Jewelry designers may use hackmanite in various forms, such as rings, pendants, and earrings, to showcase its unique color-changing ability.

Lighting and X-ray Imaging: Hackmanite's luminescence (glowing in the dark after exposure to light) has potential applications in specialty lighting and X-ray imaging advancements.

Collector's Gemstone: Due to its rarity and distinctive optical property, hackmanite is often collected by gemstone enthusiasts and collectors. The ability to observe the color transformation in response to light adds to its appeal.

UV Radiation Detector: Scientists are developing hackmanite mixtures that change color based on the type and amount of UV radiation they are exposed to. This could lead to wearable UV detectors like bracelets that warn users of sun exposure.

Metaphysical and Spiritual Uses: Some individuals believe in the metaphysical and spiritual properties of gemstones. Hackmanite, with its unique color-changing feature, may be associated with properties such as adaptability, transformation, and balance. It is sometimes used in practices like crystal healing and meditation.

UV-Activated Art and Design: Artists and designers may incorporate hackmanite into UV-activated artwork or designs. The color change triggered by UV light can be used creatively in various artistic projects.

Educational Purposes: Hackmanite's tenebrescence provides an interesting educational tool for teaching about mineral properties and optical phenomena. It can be used in classrooms and museums to demonstrate the effects of light on certain minerals.

It's important to note that while hackmanite has these interesting features, it may not be as well-known or as commercially widespread as some other gemstones. Additionally, the intensity and duration of the color change in hackmanite can vary, and prolonged exposure to light may cause the effect to diminish over time.

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