is a process that can be described as a change of color
upon exposure to light. A mineral that possesses
this property is said to be photochromic. The phenomenon was discovered in the late 1880s, including work
by Markwald, who studied the reversible change of color of
tetrachloronaphthalen. He labeled
this phenomenon "phototropy", and this name was used until the 1950s
when Yehuda Hirshberg, of the Weizmann Institute of Science in Israel, proposed the term "photochromism".
Technically, photochromism is defined as "the reversible transformation of a chemical
species between two forms by the absorption of electromagnetic
radiation, where the two forms have different absorption spectra." But
the term has been loosely adopted in mineralogical circles
to mean the reversible change of color of a mineral
upon exposure to a particular type of light.
also means the reversible change of color of a
mineral upon exposure to a particular type of light. Tenebresence is from
the Latin "tenebrae" for shadows. 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.
seems the term tenebrescence is being replaced by photochromism.
Whether you call a mineral tenebrescent or photochromic,
they both roughly mean the same thing. This effect can be repeated indefinitely, but is destroyed by heating.
Tenebrescent minerals include the Hackmanite variety
of Sodalite, some Scapolites and Tugtupite.
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
minerals may change color or lose color when exposed
to certain types of light but the change is permanent. These minerals
may also be called "photochromic" by
some. Joel Arem, in the Color
Encyclopedia of Gemstones, describes Proustite
as a photochromic gem because it can permanently
turn dark or even black with prolonged exposure to light.
This is also true of Pyrargyrite. Both minerals suffer
from this effect because of their silver content. Silver was used in
early photographic processing for this reason.
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".