Luminescence is the phenomenon that shows itself as a "glowing" of a gemstone. This is caused by absorption 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 energy comes from light, it is referred to as photoluminescence.
In gemology we are usually only concerned with the following types of luminescence:
- Tenebrescence (not technically a luminescence)
The causes of luminescence are varied, but are mostly due to impurities ("activators") or due to defects in the crystal lattice. In general, the presence of iron inside the gemstone kills or suppresses luminescence.
Fluorescence is the emission of visible light by a gemstone when exposed to a light source whose light we normally cannot see. When the gemstone is exposed to ultraviolet light (UV), which falls outside the range of light that we can see, the UV light is absorbed by the gemstone. Due to processes inside the gemstone, it will lose energy. This loss of 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 because the slowing down results in shorter wavelengths, not longer ones. See refraction).
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 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, this results in 1.8eV, hence red light (4 - 2.2 = 1.8).
There is no actual "loss" of the energy, it is converted to other kinds of energy like heat.
This might best be explained with a ball that gets tossed upward onto a staircase.
If you threw the ball up onto the stairs, that motion would require energy (the energy coming from your arm). Let's say that the ball now carries 4eV of energy and this is just enough to get it from the ground state (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 ground state 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 which 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 continuous 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 lose 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.
For day-to-day use, we use two 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 particularly true for S-UV.
Some colors that might be seen in a UV viewing cabinet:
|Ivory||Synth. white Spinel||White|
|Nat. blue Sapphire|
|Diamond||Synth. white Spinel||Blue|
Crossed filters technique
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 luckily eliminated by the invention of blue LED pocket (or keychain) torches that may be purchased 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 Color Filter. Other sheets like plastics could also serve as crossed filters.
With a sheet of blue material in front of one'slightsource, one can mimic the copper 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.
|Relationship between wavelength and quantum energy|
|<-------- wavelength --------|
|--------- energy --------->|
Phosphorescence is similar to fluorescence but differs in the "lifetime" the glow fades. In Phosphorescent materials the glow, or better the "after-glow" can range from a fraction of a second to several hours (although the latter is usually not observed in minerals).
When an electron gets enough energy from an energy source as UV light, the electron will jump from its groundstate (1) to a higher energy level (4). The electron will fall into a gap (2) rather than to the energy level just below 4 (3).
As a result the electron will be "trapped" in the gap and needs extra energy (also provided by the UV light source) to jump out. While the UV energy source feeds the electrons, fluorescence is seen, yet when the source is shutdown the electrons stay in the trap till other energy is provided to free them.
The energy required to get the electron out of the trap comes from white light (with thermal energy, or heat at room temperature).
As white light doesn't carry as much energy as UV light, the release of the electron goes at a slower rate, thus creating an after-glow. During this period the phenomenon is named phosphorescence.
One can look at the freeing of the electron as "bleaching".
Tenebrescence (see below) can be explained by this as well, yet the energy required to free the electron is higher. You can visualize it as a deeper gap if you will.
Triboluminescence is caused by pressure, friction or mechanical stress in any way applied to a gemstone.
Opposed to fluorescence, phosphorescence and tenebrescence, triboluminescence is not caused by light, hence it is not to be named photoluminscence. Instead, it is the result of electric charges, therefore an electroluminescence.
This effect is usually seen in diamond cutting. When the diamond is sawn or cleaved, electric charges break free from the stone and immediately recombine, showing a red or blue glow.
Although tenebrescence is not technically a luminescence, it shares some common characteristics with phosphorescence. Technically, tenebrescence is an unstable color caused by low energy artificial irradiation (forming a color center) from the UV light source.
The main difference between luminescence and tenebrescence is that a fluorescent or phosphorescent stone will glow in the dark, while a tenebrescent gem needs light to show its color. More technically: a tenebrescent stone needs an external energy source (light) to show its color, while luminescence is a release of "stored" energy.
In 1896, a vibrant pink variety of sodalite was discovered in Greenland by L.C. Boergstroem. The pink color of this unusual sodalite faded to colorless when exposed to light. The sodalite will return to its original pink color when it is placed in the dark for an extended period of time, or when exposed to short wave ultraviolet light. This transformation can be repeated endlessly. Tenebrescence is defined by minerals that are able to make this color transformation; minerals that display the ability to change color in this fashion are termed tenebrescent. 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 that 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.
Other examples of minerals that lose or gain color when exposed to light are:
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 under strong sunlight (but not artificial light).
Spodumene will achieve a darkening of color to pink or purple with exposure to high-energy radiation.
Chameleon diamonds are olive colored diamonds that temporarily change color after having been stored in darkness or when gently heated. Chameleon diamonds display hues and tones from light to dark olive (the stable color phase) through light to medium yellow (the unstable color phase). After one to two days in darkness, exposure to light changes the color of a chameleon diamond from the unstable yellow color back to the stable olive . 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, but in this case the loss of color is irreversible.
White barite from the Gaskin Mine in Pope County, Illinois will change to blue, and yellow barite will change to to gray-green when exposed to ultraviolet light.
The pink color of hackmanite may be restored in two ways. One way is by leaving the specimen in the dark for a few hours to several weeks, and the other way is by exposure to ultraviolet. Short-wave ultraviolet is the most efficient for this purpose. The speed with which this is accomplished and the depth of the color achieved varies from specimen to specimen.
In some specimens, long exposure to ultraviolet light is required to produce a faint degree of pink color. In other specimens, exposure to short-wave 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 then 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 at least partially the cause for the tenebrescence in hackmanite. The term F-Centers is derived from the German word Farbe, meaning color. An F-center is a defect in an ionic lattice that 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. (Nassau, 1983) 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), causing the mineral to have a different color when the lights of the UV light source 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.
A fairly recent find (2005) in Badakhshan, Afghanistan is tenebrescent scapolite. This colorless to silvery material is unearthed near the hackmanite deposits and shows an aquamarine color after exposure to SW UV light. The intensity of this color (blue) depends on the length of time it has been exposed to the UV lighting.
Exposure to a UVP UVG4 SW UV lamp for 15 minutes triggered an almost Santa Maria aquamarine blue color that faded gradually during the following 10-15 minutes in natural daylight.
|Tenebrescence effect in Scapolite|
|Photos courtesy of The Gem Trader|
Cathodoluminescence is another type of luminescence which is of some importance in gemology. It is the emission of energy , which may have a characteristic spectrum by materials which are in an electron beam. The most familiar form of cathodoluminescence (often abbreviated CL ) is the light emitted by the screen of a video monitor. If you look at the red green and blue dots you are seeing characteristic spectra of three different phosphors. In a monochrome monitor which may come in almost any color including white it is a single phosphor being excited in the electron beam.
CL is excited in special CL chambers and also in electron microscopes which already have the electron beam handy. It has long been used by mineralogists. The electron beam is higher in energy than short wave UV or X rays all of which can be used to excite more conventional fluorescence. Many synthetic diamonds show characteristic growth lines under cathodoluminescent examination. It is an advanced or research gemology technique.
Fluorite can store energy from UV radiation and when the mineral is heated, it will release part that energy through luminescence.
- Gemology - Peter Read, 3rd edition (2005) ISBN 0750664495
- Gem-A Diploma Syllabus (1987)
- Crossed Filters revisited - D.B.Hoover and B. Williams, The Journal of Gemmology, July/October 2005
- A Status Report on Gemstones From Afghanistan - Gems & Gemology, Winter 1985, Gary Bowersox
- Update on Hackmanite - Gems & Gemology, Winter 1989, Gem News
- The Physics & Chemistry of Color - Kurt Nassau, 1983
- An Introduction to Rock Forming Minerals - Deer, Howie & Zussman 1966
- Hackmanite - Brochure supplied by SoCalNevada, on Hackmanite from the Kola Peninsula, Russia
- The origins of color in minerals - Kurt Nassau, American Mineralogist Volume 63, pages 219-229, 1978