Luminescence

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Luminescence is the phenomenon that shows itself as a "glowing" of a gemstone. This is caused by absorbtion of energy and the releasing of surplus of this energy in small amounts.
The sources of energy are usually ultraviolet light, X-ray light and even visible light. When the emitted energy produces light that falls in the visible range of the spectrum, it is refered to as photoluminescence.

In gemology we are usually only concerened with 4 main types of luminescence:

  • Fluorescence
  • Phosphorescence
  • Tenebrescence
  • Triboluminescence

The causes of luminescence are varied, but are mostly due to impurities ("activators") or due to the molecular packing of the crystal lattice. In general the presence of iron inside the gemstone kills or surpresses luminescence.


Fluorescence

Basic

Fluorescence is the emission of visible light by a gemstone when exposed to a lightsource whose light we normally can not see. When the gemstone is exposed to ultraviolet light (UV), which falls outside the range of light which we can see, the UV light is absorbed by the gemstone and due to processes inside the gemstone it will loose energy. This loss in energy causes the UV light to change to a color in the visible light range (red, orange, yellow, green, blue, indigo or violet).
Although some people understand this as a speeding up of light, this is incorrect (neither is it caused by the slowing down of light inside the gemstone).

All rays of light carry a specific amount of energy. Light with a lower wavelength has higher energy. This energy is expressed in eV (electron Volts). For instance red light has an energy of around 1.8eV while violet light has 3.1eV of energy.
When the loss of energy of UV light (with an energy of let's say 4eV) is 2.2eV, that would result in 1.8eV and thus red light (4 - 2.2 = 1.8).

Fig.1 Simplyfied diagram showing cause of fluorescence

This might best be explained with a ball that gets thrown on a staircase.
If you would throw a ball up the stairs that would require energy (the energy coming from your arm). Lets say that the ball now carries 4eV of energy and this is just enough to get it from the groundstate (1) to the 3rd board (4). As it then drops from level 4 to level 3, it would loose part of that energy (in this example 0.5eV). So the ball still has 3.5eV of energy. It will then drop to level 2, loosing an additional 0.3eV of energy. After this it will drop down to the ground again while carrying only 1.8eV of energy. When it reaches the groundstate again, the ball looses all its surplus energy.

Now imagine the ball being an electron and the source of energy (formerly your arm) is ultraviolet light. As the electron gets 4eV of energy from the UV light source we can't see it as light (we can only see it as it reaches 3.1eV, which corresponds to violet light), consequently it will loose more and more energy as it drops down level by level. When it reaches level 2, it has an energy of 1.8eV and this corresponds with red light, so the electron will now emit red light.

As long as energy is fed to the electrons (in the form of UV light), this process is continious and this process only takes a fraction of a second (a femtosecond or 10-15 seconds). How much energy that is required for a gemstone to fluoresce varies from stone to stone, for Ruby that is 3eV and explains why the best Rubies appear to glow like hot coil in daylight.
Not all gemstones will show this phenomenon and those gemstone loose the extra energy in another way.

The fluorescence lifespan is relative to the UV light source, meaning that if you turn off the lightsource the fluorescence is gone.

The electromagnetic spectrum and the place of ultraviolet light


For day to day use, we use 2 different types of UV light:

  • Shortwave ultraviolet light, or S-UV (with a wavelength of about 254nm)
  • Longwave ultraviolet light, or L-UV (with a wavelength of about 366nm)

Warning: When using UV light, make sure to protect your eyes as they are damaging! This is particulary true for S-UV.


Some colors that might be seen in a UV viewing cabinet:

Fluorescence
L-UV S-UV Produced color
Ivory Synth. white Spinel White
Opal
Ruby Red
Red Spinel
Synth. Emerald
Nat. blue Sapphire
Alexandrite
Diamond Synth. white Spinel Blue
Moonstone
Apatite Green
Fluorite Violet
Kunzite Orange
Lapis lazuli
Sodalite
Zircon Yellow
Topaz

Advanced

Crossed filters technique

Copper Sulphate solution in a flask and a red filter

The "crossed filters" technique should not be confused with "crossed polars" or "crossed polaroids" as they have to do with polarization, not luminescence.
A flask is filled with hydrous copper sulphate and white light is being passed through the solution. The exiting light will be blue. During the illumination of the gemstone with this blue light, a red filter is placed between the eye of the observer and the stone. When the stone appears red, when viewed through the red filter, this is clear proof that the stone is fluorescent in daylight.
The activator in the gem which causes this is the presence of Chromium (Cr) in the crystal lattice and this effect is predominantly seen in Ruby, Alexandrite, Emerald, red Spinel and pink Topaz. It should be noted that Iron (Fe) can greatly diminish or completely eliminate this fluorescence effect. As synthetic materials usually carry more Cr and little to none Fe, this glowing of red light is more intense than in their natural counterparts (in general).

The hassle of carrying hydrous copper sulphate is luckely eliminated by the invention of blue LED pocket (or keychain) torches which can be obtained for just a few USD at your local hardware shop. One could use a sheet of red selenium glass as the red filter, or even your Chelsea Colour Filter. Other sheets like plastics could also serve as crossed filters.
With a sheet of blue material infront of your lightsource one can mimic the cupper sulphate solution and/or the LED torch.

Using the same lightsource in conjunction with a spectroscope, one can then easily distinguish between Ruby and red Spinel.


Jablonski energy diagram

Fig.2 Jablonksi energy diagram

in Fig.2 "s0" consists of 3 vibrational levels and "s1" of 4. The "s" stands for "singlet state", meaning that all electrons are spin-paired.

An absorbed photon will cause an electron to be excited from the groundstate (s0) to higher energy levels (s1 or higher). As the electron drops from the higher vibrational states in s1 to the lower s1 levels, it looses energy through internal conversion. After relaxation in s1 (at a timerate of 10-12 seconds) it will then drop to groundstate s0 with lower energy (10-9 seconds).

Due to the fact that quantum energy is opposite proportional to wavelength, the emitted photon (which carries lower energy as the absorbed photon) must travel at a longer wavelength. This difference in wavelengths is known as Stokes Shift.

Relationship between wavelength and quantum energy
<-------- wavelength --------
wavelength 800nm 700nm 600nm 500nm 400nm 300nm 200nm
energy 1.55eV 1.77eV 2.07eV 2.48eV 3.1eV 4.13eV 6.2eV
--------- energy --------->


Phosphorescence

Basic

The "trap" diagram


Advanced

Jablonski energy diagram (phosphorescence)


Tenebrescence

In the early 1800's. a vibrant pink variety of sodalite was disovered in Greenland. The pink faded rapidly to colorless when exposed to light. When placed in the dark, the sodalite recovered the pink color. The alteration between pink and colorless could be repeated again and again. Minerals that are capable of such color change are said to be tenebrescent, from a Latin word referring to darkness. Tenebrescence is the property that some minerals and phosphors show of darkening in response to radiation of one wavelength and then reversibly bleaching on exposure to a different wavelength. Very few minerals exhibit this phenomenon, also known as reversible photochromism, a word that applies to sunglasses which change color density on exposure to sunlight.

SODALITE that shows this behavior has been given the variety name Hackmanite. The pink color in this mineral is unstable because it fades very quickly when exposed to light. There are other examples of minerals that lose or gain color when exposed to light:

TUGTUPITE, some light colored varieties of tugtupite, especially pale pink material, will intensify in color as a result of exposure to shortwave UV—or even strong sunlight (but not artificial light).
CHAMELEON DIAMONDS are olive colored diamonds that temporarily change color after having been stored in darkness or when gently heated. Chameleon diamonds exhibit a range of hues and tones from light to dark olive (stable color phase) through light to medium yellow (unstable color phase). After 24-48 hours in darkness, exposure to light gradually changes the color of a chameleon diamond from the unstable yellow phase back to the stable olive phase. This is observed as an infinitely repeatable process.
AMETHYSTS from Globe, Arizona, and some SHERRY-COLORED TOPAZ are reported to loose their color in the sun. This loss of color is irreversible.
WHITE BARITE from the Gaskin Mine in Pope Cunty Illinoios, will change to blue, and yellow barite to grey-green when exposed to ultraviolet light.


The pink color of hackmanite may be restored in two different ways. One is by leaving the specimen in the dark for a few hours to several weeks, or, exposure to short-or long wave ultraviolet will also restore color. Short wave ultraviolet is the most efficient for this purpose. The speed with which this is accomplished and the depth of the color acheived varies from specimen to specimen.

Hackmaninte1.jpgHackmaninte3.jpgHackmaninte2.jpgHackmaninte4.jpg
Hackmanite quickly fades to colorless when exposed to light. The original color
can be restored quickly by exposure to short-wave ultraviolet light, or more slowly
by storing it in a dark space.

In some specimens, long exposure to ultraviolet light is required to produce a faint degree of pink color. In other specimens, exposure to shortwave ultraviolet will almost instantly produce a pink color. In the latter specimens, additional exposure to ultraviolet light for several minutes to a few hours will produce a deep pink to raspberry-red color in which a weak blue component is evident. This can be seen in some specimens from Mont Saint-Hilaire and Khibina. If the specimen is now put in the dark, the deep red color will exhibit phosphorescence also known as "after glow". Visible light (wavelengths between 480-720 nanometers) will quickly reverse the process and render the specimen colorless once again.

This photochromic effect can be repeated indefinitely, although any heating of the mineral destroys tenebrescence forever.

Research indicates that F-Centers are the cause, at least partially, for the tenebrescence in hackmanite. The term F-Centers comes form the German word Farbe, which means color. An F-center is a defect in an ionic lattice which occurs when an anion leaves as a neutral species, leaving a cavity and a negative charge behind. This negative charge is then shared by the neighboring positive charges in the lattice. F-Centers are responsible for coloring a variety of minerals, including fluorite and barite. In hackmanite, it is proposed that some of the negatively charged chlorine atoms are missing. A negative electric charge is required at such vacancies to provide charge balance, and any free electrons in the vicinity become drawn to such vacancies and are trapped there. Such a trapped electron is the typical basis of an F-Center. It appears that this center in hackmanite absorbs green, yellow, and orange light and varying amounts of blue. When the hackmanite is seen in white light, red and some blue are returned to the eye, giving the hackmanite colors.

A mineral may produce a certain color that depends on different, but fixed arrangements of electrons (Nassau, 1983). Hackmanite absorbs the energy from the ultraviolet radiation and many electrons get stuck in a new, high-energy position in atoms (F-centers); this is what causes the mineral to have a different color when the lights are turned on. But when we turn the room lights on, the new color fades. White light (the visible spectrum) also energizes electrons, just not as much as ultraviolet light. The white light has the necessary energy to unstick the electrons from the F-Centers, thus returning the mineral to colorless.

Triboluminescence

Sources

  • Gemology - Peter Read, 3rd edition (2005)
  • Gem-A Diploma Syllabus (1987)
  • Crossed Filters revisited - D.B.Hoover and B. Williams, The Journal of Gemmology, July/October 2005

External links