When a mineral or other material converts energy of any wavelength into an emission of light waves of a longer wavelength, it is said to be FLUORESCENT. Apparently, this occurs when some of the electrons within certain atoms are raised to a higher energy level by the energy received and, in returning to their former level, give off a visible luminescence. This phenomenon is noted most often when invisible ultraviolet light is directed at a gem material and when the new wavelength given off is in the visible spectrum. When this happens, the invisible light produces a glow within the gemstone that we call Fluorescence. If the glow continues after the energy source is turned off, the result is called PHOSPHORESCENCE.
Fluorescence is not limited to the conversion of invisible light (i.e., ultraviolet and X-rays) into wavelengths of the visible spectrum. Visible light causes fluorescence, too, and the new light given off by the stone also is in the visible spectrum, although it is of a longer wavelength. For example, the strongest red fluorescence of ruby or synthetic ruby is excited by a wavelength of approximately 5100 Angstrom units (one Angstrom unit = one ten-millionth of a millimeter), which is in the green portion of the visible spectrum. Light near the edge of the visible violet portion of the spectrum is particularly effective in exciting blue fluorescence in diamond.
Unfortunately, not all gem materials fluoresce; even those that do react do not always exhibit a sufficiently characteristic color or strength to be of great value-in identification. However, there are some important exceptions. This assignment includes a table of fluorescent colors in gem materials that have been observed in the Laboratories. A portion of the body of the assignment discusses those that have the most importance to the gemologist. The table indicates the dependability of the fluorescence described; in other words, the phenomenon may be caused by a coloring agent or other impurity that is not always present in every specimen of a given species. When this is the case, the strength and color of the fluorescence is not entirely reliable. For example, it may vary considerably in many of the allochromatic gems i.e., those colored by unessential constituents), depending on the strength of fluorescence of the coloring agent or other impurity. Colorless synthetic and natural corundum react very weakly, but both synthetic and natural ruby usually are strongly fluorescent, although generally the intensity is distinctly different. Some natural yellow sapphires fluoresce; the synthetic is either inert or reacts very weakly. Most blue synthetic sapphires fluoresce, but it is somewhat difficult to detect. Thus, it is impossible to say without qualification that fluorescence is characteristic of corundum. Its diagnostic value in this species depends on the specimen under consideration and its mode of formation. Since the majority of colorless, transparent gemstones are non-fluorescent, the fluorescence they may show depends on impurities; therefore, it varies greatly in most allochromatic gem species.
It is essential that the gem man understand the many factors that can change the appearance of a stone under ultra-violet radiation. Variations such as those described are not the only ones; those caused by source and filters are explained later in the assignment.
In spite of all this, fluorescence may be a valuable test when other means provide insufficient information or require more time than is available, or when ultraviolet light is the only means of testing at one's disposal. It may be particularly useful, for example, in separating large numbers of synthetic from natural stones. There is a distinct difference in the intensity of fluorescence between synthetic and natural ruby, synthetic and natural emerald, and synthetic and natural blue sapphire. As a consequence, when known stones are available for comparison purposes, it is possible to effect a rapid separation of large numbers of stones. Since this test is somewhat subjective, it must be used with great care, preferably with known specimens for comparison. Actually, it should be supplemented with other tests, such as magnification. It is nevertheless a useful tool for the gemologist.
THE COMMON SOURCES OF ULTRAVIOLET LIGHT
The visible-light spectrum extends from approximately 4000 A.U. at the violet end to approximately 7000 A.U., the wavelength that divides red from infrared light. The wavelength range visible to humans varies considerably; some are able to see wavelengths that are invisible to others. For example, some persons do not see light of 4000 A.U., but have a cutoff point substantially above that figure. Others may be unable to see light of a wavelength measuring 7000 A.U., but perhaps only to 6800 or 6900 A.U. The usual light sources for the excitation of fluorescence are in the ultra-violet portion of the spectrum; that is, of less than 4000 A.U.
The usual ultraviolet light sources are mercury-vapor lamps. Commercial lamps used in gem testing range from small 4 watt models to 100 watt lamps. The advantage of the stronger is the more intense fluorescent reactions; however, the small 4 watt unit is generally recommended because it is convenient to handle. The unit pictured is available in both short and long wavelength types. Lamps of lesser intensity, frequently featured in hobby kits, should not be used for analyzing gemstones, because the reactions are generally so weak as to be inconclusive.
A mercury-vapor lamp consists of a tube of very pure silica glass filled-with argon gas and a small amount of pure mercury. Commercial glass cannot be used for any ultraviolet application, since it does not transmit ultraviolet; quartz or pure silica glass does, however, a single electrode extends into one end of the tube and at the other end a similar main-electrode is accompanied by a smaller starter electrode. A ballast supplies sufficient voltage for starting the lamp. When the ballast circuit is started, the full starting voltage is between the starting electrode and the main electrode, which draws some electrons across the gap; this starts a glow discharge between them. The resistance in the starting circuit limits the current to a very low level (a few milleamperes), ionizing enough argon gas to reduce the resistance between the two main electrodes. Ultimately, an are jumps across these electrodes and the heat from it vaporizes the mercury.
Mercury arcs give off a number of lines in the ultraviolet portion of the spectrum; these include 2537 and 3654 A.U., as two of the more prominent emission lines. Ultraviolet radiation is achieved by introducing a filter that absorbs most of the emission in the four wavelengths in the visible spectrum to which mercury radiation is confined. Another factor in controlling the emission of different wavelengths is the vapor pressure of the mercury. This is determined by the volume of the tube, the amount of mercury added, and the current. Two different filters are used: one that transmits maximum radiation in the 2200 to 2800 A.U. zone and another in the 3200 to 4000 A.U. zone. The lamps that produce the 2537 A.U., or short, ultraviolet (also called far ultraviolet) differ from those that produce long, or near, ultraviolet in several ways. They do not have a phosphor coating and the glass tube will transmit far ultra-violet. The filters differ in another way. The filter used on the near ultraviolet lamp is sufficiently stable to be useful for many years, whereas the filter on the far-ultraviolet lamp has a life of only approximately one year or four hundred hours of use.
The accompanying tables show the usual reactions under long-and short-wave ultraviolet lamps and X-rays, It must be understood, however, that the results mast vary considerably, depending on a number of factors. One is the effectiveness of the filter, and another is the strength and balance of the source. For example, a small, low-wattage hand unit may give different results than a large unit of higher wattage. Slight differences in the ability of the filters to transmit visible light will affect the visibility of fluorescence. If the short-wave ultraviolet is filtered by a glass that has deteriorated; the effect is likely to be very similar to that expected from long-wave ultraviolet. If a long-wave source is strong and the filter not too effective, a significant visible-radiation results and may cause distinctly different fluorescent effects for a given species than those shown in the table. With so many variables, it is difficult to predict results. In addition, the fluorescence of the two stones of the same species that have approximately the same color to the eye may vary widely under the same light, because of a difference in the nature of the coloring agent and its tendency to cause fluorescence.
Despite all of the possible variations, some reactions are sufficiently characteristic to be useful in identification. Some of these reactions are rarely used, however, because other tests are more practical. Fluorescence is used in a fully equipped laboratory in fewer separations than is likely when other equipment is limited. There are a number of gems that may be identified by a combination of fluorescence and only one or two other check tests.
IMPORTANT FLUORESCENT TESTS
Determining the origin of color of black pearls calls for the-use of ultraviolet light. Under a strong source of long-wave ultra-violet, most natural black pearls exhibit a reddish fluorescence. It may range from a light pink, rarely mottled pink and white, to a dark cherry red. Dyed black pearls either are inert or show a very faint whitish glow. Examination should be made in a darkened room.
Synthetic From Natural Emerald
Prior to recent discoveries, a helpful test in the separation of natural and synthetic emeralds was fluorescence under longwave ultraviolet light. Natural emeralds are most often inert, but fine qualities may exhibit a weak orangey-red to violetish-red. However there is now a group or Gilson synthetic's that are also inert, and as all the other-types of synthetic emeralds are known to exhibit characteristic fluorescence, care must be used in application of this test. Under longwave ultraviolet light, the Chathams fluoresce brick red, certain groups of Gilsons vary from a strong orangey-red to red, and the Linde Hydrothermals a strong red.
The strength of fluorescence exhibited by the new synthetic emerald coated beryl depends on the thickness of the coating over the natural core. When the coating is thick, the fluorescence is distinct, but not strong enough to be of much value in testing. On thinly coated stones it is often difficult to detect.
Synthetic From Natural Ruby
Most synthetic and natural rubies react so strongly to ultraviolet radiation that they are classed among the most fluorescent of gemstones. However, there is a difference in the fluorescence, which is sometimes useful when attempting to separate calibre cut stones. If it is used in an effort to separate a single natural from a single synthetic ruby, it is necessary to have comparison stones of similar color and known identity.
The fluorescence of natural ruby varies from the very strong red glow of the Burma variety to the very weak reaction of Siamese stones. The strength of fluorescence in a synthetic ruby is sufficiently greater that if it is compared side by side with a natural, the glow will be slightly more intense in the manmade stone under both long and short wave ultraviolet and X-rays. Fluorescence under X-rays is useful to some extent in identification, since the synthetic ruby phosphoresces after the radiation is turned off and the natural does not. Of course, this is visible only when the eyes have become accustomed to the dark and when the X-ray unit is used in a darkened room.
Synthetic From Natural Blue Sapphire
Both synthetic and natural medium to dark blue sapphire are inert to long wave ultraviolet. Under short wave radiation, the synthetic assumes an appearance similar to that if the stone were sprinkled liberally with a chalky yellowish-green powder. The natural remains inert under short wave ultraviolet. WARNING: THIS TEST REQUIRES COMPLETE DARKESS. Exposing a tray of synthetic blue sapphires to short wave ultraviolet reveals the characteristic fluorescence in a number of the stones very readily, but in others it is not visible at first glance or even upon careful scrutiny. These stones that react weakly must be removed from the group and placed on a dark background for re-examination. If the eye is totally accustomed to the darkness and the lamp is fresh and strong, the fluorescence should be visible. Since it is easy to make a mistake when performing this test, it is to be regarded as accurate only when the fluorescence is distinctly visible, when the stones may be safely relegated to a synthetic category. Complete absence of fluorescence should not be regarded as proof of natural origin. This would be a dangerous assumption, since the glow is not strong and could be overlooked easily.
Natural Yellow From Synthetic Yellow Sapphire
Fluorescence occasionally is useful in distinguishing natural from synthetic yellow sapphire. The synthetic of a dark tone or orange-yellow cast may be expected to show the following fluorescence: brick red under long wavelength, orangey red under short wavelength, or very weak yellow-orange under either wave length. In some instances they may be inert, particularly the lighter tones. Those from Cevirsn exhibit an orangey-yellow fluorescence under long wave ultraviolet, the strength of which is in direct proportion to the depth of the body color. Under short wave radiation, the fluorescence is weak yellow-orange. Yellow Australian sapphires do not fluoresce.
Another Use For Ultraviolet Light
Natural gemstones and their synthetic counterparts may show differences in their transparency to ultraviolet radiation. Most natural transparent corundum becomes opaque to short-wave radiation fairly close to the point at which the human eye can detect light. In other words, the cut-off point of their ultraviolet transmission is less than one thousand Angstrom units below the cut-off point of human visibility.
Since the transmission of the short-wave ultraviolet lamp is concentrated in the area of 2537 A.U., usually it is transmitted by the synthetic but not by the natural. This may be tested for by using the short-wave lamp and directing its light through the the stone placed over an opaque shield into which a small hole has been cut. Below the hole is placed a phosphor that will fluoresce only to short-wave ultraviolet. If the unknown transmits the short-wave radiation, the phosphor will light up, if it fails to transmit the radiation, it will remain dark. Of course, this test must be performed in a darkened room, and the gemologist must be certain the phosphor is directly below the opening. The phosphor used for this purpose is the mineral scheelite (calcium tungstate, the most important ore of tungsten). It is non fluorescent under long-wave ultraviolet, but very strongly fluorescent in a bright-blue color under short wave radiation.
This test also may be performed by placing a piece of contact printing film, emulsion side up, in a flat-bottomed dish and placing the stones on the paper covered with water. This must be done in a darkened room. Then a short wave ultraviolet lamp is turned on for a very brief period, to determine whether the light passes through the center of the stone and exposes the firm slow printing paper, such-as Kodak Velox, is recommended over fast bromide papers. Stones that are opaque to short wave ultraviolet are seen as white spots, whereas the centers are exposed and appear as dark spots on those that are transparent to short wave ultraviolet. Because some of the exposure is due to the visible light emitted from the lamp, the test is accurate only when minimum exposures are used and known stones of similar size to the one being tested are used for comparison.
This test is no longer reliable for separating natural from synthetic emeralds, since in both, opaque and transparent reactions are encountered. Also, when testing for transparency to X-rays, both naturals and most synthetics are transparent with the exception of the group of non fluorescent Gilsons. When tested with X-ray, these stones are opaque. The opaqueness of this group of Gilsons is apparently due to the addition of iron which also accounts for its lack of fluorescence, and its increased refractive index and specific gravity. The presence of iron also gives this stone a characteristic absorption spectrum with a line at 4270 A.U., and makes testing with the spectroscope very important in its identification.
Ultraviolet Fluorescence Chart
Abbreviations used in this chart:
|lw||long wavelength fluorescence|
|sw||short wavelength fluorescence|
FACTORS THAT MAY CAUSE DIFFERENCES, IN FLUORESCENCE:
- Chemical impurities
- Matrix and / or other foreign materials combined in or on this material.
- Strain in the structure of the material
- Isomorphous replacement
Please-note that it may be necessary to raise the stone up to the ultraviolet unit itself to obtain the required intensity of ultraviolet radiation necessary to produce the fluorescence indicated for a particular stone.
|Amber||None to yellowish green to orangey-yellow (lw) (lw works best). Also may fluoresce white, yellow, green or blue (lw), Fluorescence generally weaker under sw.|
|Beryl||Emerald: (lw works best) none but may
fluoresce weak orangey-red to weak red in very fine colors
(lw and sw).
Oiled emerald: Oil shows yellowish to orangey-yellow(lw); weaker to non(sw). Oil will be noted in fractures.
Blue Beryl: Green (sw).
Colorless: None to pale yellow or pink (lw and sw).
Morganite: None to weak light red to violetish red (lw and sw).
|Synthetic Emerald||Most synthetic emerald fluoresces a dull brick red (lw and sw) (lw works best). Some Gilsons fluoresce a weak to moderate orangey-red (lw and sw) to a moderate yellowish green (lw and sw). Gilson Group-III is inert.|
|Chalcedony||All varieties: Generally inert (lw and sw); however, some may fluoresce a weak to intense yellowish green (lw and sw).|
|Chrysoberyl||Alexandrite: None to weak red (lw and
Yellow and greenish yellow: None to yellowish green (sw).
Other colors: Generally inert.
|Coral||White: None to weak to strong bluish
white (lw and sw).
Light and dark shades of orange, red and pink: None to orange to pinkish orange (lw and sw).
Ox blood: None to dull deep red (lw and sw).
|Corundum||Burma ruby: Strong red (lw); moderate
Ceylon ruby: Strong orange-red (lw); moderate orange- red (sw).
Siam ruby: Weak-red (lw); weak red to none (sw).
Pink sapphire: Strong orange-red (lw); weak orangey-red (sw).
Orange sapphire: None to strong orange-red (lw). Ceylong yellow sapphire: Moderate orange-red to
orange-yellow (lw); weak red to yellow-orange(sw). Green sapphire: None.
Ceylon blue sapphire: Moderate to strong orange to red (lw); weaker (sw).
Violet and alexandrite-like sapphire: None to moderate to strong red (lw); weaker (sw).
White sapphire: None to weak to strong orange to orange-red (lw and sw).
Brown sapphire : None to weak red (lw and sw). Black sapphire: None.
Colorless sapphire: Orange-red to red to orange (lw and sw).
Blue (dark) sapphire: None to moderate red (lw and sw).
|Synthetic Corundum||Synthetic ruby (flame fusion): Very
strong orangey-red(lw); moderate to Strong orangey-red (sw).
Synthetic ruby (flux fusion): Strong orangey-red (strong, but not quite as strong as flame fusion or highly fluorescent natural material) (lw); orangey-red fluorescence generally stronger than natural, but some material may show zoned area of blue and /or bluish over tint on orangey-red fluorescence (sw).
Synthetic orange sapphire: Very weak orange to red (sw).
Synthetic yellow sapphire: Very weak red (sw).
Synthetic green sapphire: Weak orange (lw); dull brownish red (sw).
Synthetic blue sapphire: Weak to moderate chalky blue to yellowish green (sw).
Synthetic violet sapphire: Strong red (lw); greenish blue (sw).
Synthetic alexandrite-like sapphire: Moderate orange to red (lw and sw); may fluoresce red (lw), mottled blue (sw).
Synthetic colorless sapphire: None to weak bluish white (sw).
Synthetic brown sapphire: None to weak red (lw and sw).
Synthetic pink sapphire: Moderate to strong red (lw); pinkish violet (sw).
|Diamond||May fluoresce all colors with the exception of violet. General fluorescence is weak to strong blue (lw and sw).|
|Feldspar||Albite: None to very weak brownish red
(lw and sw).
Amazonite: None to weak yellowish green (lw). Oligoclase (sunstone): None to weak mottled white (lw and sw).
Transparent yellow orthoclase: None to weak reddish orange (lw and sw).
Orthoclase (moonstone): None to blue (lw); orange (sw). May fluoresce weak pink (lw and sw).
Transparent labradorite: None to weak blue (lw).
|Garnet||All species and varieties are inert with the exception
Transparent green grossluarite: None to moderate red (lw and sw).
Transparent colorless groosularite: None to weak orange or green (sw).
|Ivory||Weak to strong bluish white (lw and sw).|
|Jadeite||Light green: None to weak white (lw).
Light yellow: None to weak green (lw).
White : None to weak yellow (lw).
Light violet: None to weak white (lw).
Some color-treated lavender: Weak to moderate orange (lw); weaker (sw).
Dark colors: Virtually inert.
|Lapis Lazuli||Generally inert; may fluoresce weak to moderate green or yellowish green (sw). The calcite inclusion may fluoresce pink (lw)|
|Opal||Body color black or white: None to
white to moderate pale blue, green or yellow (lw and sw);
Common opal or hyalite: None to strong green or yellowish green (lw and sw); may phosphoresce.
Fire opal: None to moderate greenish brown (lw and sw) ; may phosphoresce.
|Pearl||Natural: None to strong blue, yellow,
green or pink (lw and sw).
Cultured: None to strong blue, yellow, green or pink (lw and sw).
Black pearl: Natural - none to strong red (lw); some may fluoresce light pink, mottled pink and white.
Dyed black: inert to weak white (sw).
|Quartz||Green aventurine: None to weak grayish
green (lw and sw).
Red dyed quartzite: Weak to strong red (lw and sw).
Rose quartz: None to weak red (lw and sw).
All other varieties: Virtually inert
|Shell||White: None to moderate blue to
greenish white (lw and sw).
Tortoise shell: All but dark areas dull bluish green (lw and sw).
|Spinel||Red, orange and pink: None to weak red
to orange-red (sw); weak to strong red and orange (lw).
Near colorless (rare): None to moderate orange to orange-red (lw).
All other colors: Virtually inert.
|Synthetic Spinel||Colorless: Moderate to strong chalky
blue (sw); may fluoresce weak green (lw) and strong greenish
Light blue: Weak to moderate orange (lw); chalky blue (sw).
Medium blue: Strong red (lw); strong bluish white (sw).
Dark blue: Strong red (lw); strong mottled blue (sw).
Light green: Strong yellow (lw).
Dark green: Strong violetish red (lw); strong greenish white (sw).
Alexandrite like: Moderate dull red (lw and sw).
Yellowish green: Strong yellowish green (lw and sw).
Red: Strong red (lw).
|Topaz||Colorless: None to weak yellow (lw).
Red: Weak brownish Yellow (lw).
Yellow: Weak orange yellow (lw).
Blue: None to moderate yellow (lw).
Brown: Weak orange-yellow (lw).
Pink: Moderate greenish white (sw).
|Tourmaline||Pink: None to very weak red (lw and
All other colors: Virtually inert.