Kurt Nassau, Ph.D., FGA (Hon.), of Nassau Consultants, Lebanon, N.J., has long studied and written on the treatment and synthesis of gemstones.
It’s a widely accepted principle that gemological testing used to identify the precise nature of a gemstone material should be non-destructive. Yet there are exceptions. A close examination of any gemology textbook shows that some “slightly destructive” tests are acceptable for difficult gemological distinctions where no other approach can give the answer. These include the use of scratch testing to determine hardness, acid to identify carbonates, ether to distinguish amber from some of its imitations, acid and various solvents to identify the use of dyes and the hot-point to detect amber and wax impregnations. (Of course, the tests should be done in an inconspicuous area to minimize the damage.)
Fade testing — subjecting a gemstone to light to weaken its color –is an option when a fading color and a non-fading color occur in a gemstone material with no other known test to distinguish between the two. There are two important circumstances when this occurs: yellow-orange-brown sapphire (shown in photo) and yellow-orange-brown topaz. Fading and non-fading colors are found in these naturally occurring materials. Both types of color may occur in the same stone; if exposed to strong light, the one component fades while the other doesn’t.
Both types of colors may also be produced by irradiation in attempts to induce color in colorless or pale material, to make a color permanent or to restore color after it has faded. Experience has shown, however, that if a naturally occurring color fades on exposure to light, irradiation can restore it but not make it permanent.
When we speak of fading, we mean the weakening or loss of color on exposure to light. Heat can produce loss of color also, but heat-induced changes are frequently different from light-induced changes. Because significant heat (say over 50º#-70#C or 122º#-70#F) does not occur under normal wear or display conditions, retailers generally need to consider only fading from light. (Note: The imprudent use of display cases with intense lighting combined with direct sunlight can produce excessively high temperatures; such circumstances should always be avoided.)
Sometimes there may be subtle differences between fading and non-fading colors in these materials, as in fluorescence. However, you can’t rely on such differences to indicate the presence of fading or the use of irradiation. No formal tests, simple or elaborate, are known that can distinguish fading from non-fading material in yellow-orange-brown sapphire or topaz. Neither can it be determined by any known test whether these colors derive from nature or from irradiation applied by man. From a practical point, a fading color wouldn’t likely survive the heat and light exposure involved in the faceting process. Therefore, we can assume a faceted fading gemstone of this type was probably irradiated after the faceting step.
I once suggested that “a fade test [of an irradiated gemstone] is not “destructive” of a natural color in the usual sense, but should be viewed as being “restorative” to the stable color it had before the irradiation step.” (Kurt Nassau, Gems and Gemology, #28 (3), p. 150, Fall 1992).
Sapphires: There are at least seven types of fading and non-fading yellow-orange-brown sapphires (for details see Kurt Nassau, Gemstone Enhancement, Butterworth-Heinemann, 2nd Ed., 1994, pp. 129-140). Sapphires that derive these colors from an iron impurity do not fade; those where a color center is involved may have a stable color or may fade. The presence of iron has been suggested as a proof of non-fading in such sapphires, and its absence an indication that fading can be expected. But this is not a reliable test for two reasons:
Some stable-color sapphires do not contain iron.
A weak iron-produced color could be accompanied by an irradiation-produced fading color component.
It’s customary to expose freshly mined sapphires of these colors to sunlight for several days; some may turn lighter or lose color; others may acquire color or darken. The fading and non-fading forms of these colors may be present separately or together. When fading does occur, the color will be lost after several hours of direct exposure to strong sunlight or after several days to weeks under normal wear or display. Irradiation can produce both types of color, again separately or together.
A related material is orange-pink padparadscha sapphire. This results from a weak ruby red derived from a chromium impurity combined with yellow. So a padparadscha sapphire could also have fading yellow present (either natural or irritation-induced) and could lose an important part of its color upon exposure to light.
The same fading and non-fading colors can occur also in synthetic yellow-orange-brown and padparadscha sapphires.
Topaz: The group of fading and non-fading yellow-orange-brown topaz includes “imperial topaz” and “sherry topaz.” Of course we are speaking of “precious topaz,” not the “smoky topaz” label so often erroneously used for smoky quartz; “Madeira topaz,” used for the yellow citrine variety of quartz; and so on.
Just as with sapphire, some naturally occurring topazes of these colors fade while others don’t (ibid, 187-194). Some brown topaz from Utah and Mexico fades, for example, while most such topazes do not.
The rapidity of fading is similar to that described already for sapphire. Here again, irradiation can produce both types of color, either separately or together.
Yellow-orange-brown and “imperial” or “sherry” topaz are valued gemstone materials. However, they can lose some or all of their color rapidly upon exposure to light if they are of the fading type. This change requires a few hours of continuous exposure to strong sunlight or a few days to weeks under normal wear.
Testing: Material that loses color rapidly is hardly suitable for use in jewelry. Therefore, the determination of fading should be an important part of the gemological identification process. This can’t be done at present except by resorting to a fade test itself. Inevitably, such a test would destroy at least some of the color in material of the fading type.
There is no standard way of fade testing in gemology. The technique described here, which I have used for 30 years, permits the rapid and reliable identification of rapid-fading color and has negligible effect on slowly and non-fading materials.
Material is exposed to light from an ordinary 100-watt frosted incandescent light bulb at a distance of six inches (figure 1). It’s most important that a fan keep the material close to room temperature, certainly less than 50º#-70#C. An elevated temperature may speed up the test, but that’s not advisable because heat and light have different effects on a gem. Light has no effect on golden beryl and greenish aquamarine, but a temperature as low as 100º#-70#C produces a loss of yellow in both.
In a fading apparatus (figure 2), 24-hour exposure produces strong or total fading in a rapidly fading material such as yellow-orange-brown sapphire or topaz. Don’t rely on the fallible visual memory (or before/after photos). Some type of color measurement must be performed before and after fade testing or a color-matching reference material must be used.
If significant fading is observed in 24 hours, one or two additional 24-hour exposure periods may be necessary to determine when fading is complete.
Other fading materials: Many minerals fade or have their color altered by light (see Kurt Nassau, “Conserving Light-Sensitive Minerals and Gems” in The Care and Conservation of Geological Materials, F.M. Howie, editor, Butterworth-Heinemann, 1992, pp. 11-24). But jewelers would see few of these in the course of their business. Materials fading about as rapidly as yellow-orange-brown sapphire and topaz include green irradiated kunzite (not hiddenite), green irradiated topaz (fades to blue) and green irradiated grossular. The color changes of the very rare chameleon diamonds fall into the group also.
Some materials don’t fade significantly for one to several weeks in the fade test described above and will last a few to many years under normal wear or display circumstances. They include pink kunzite, some rose quartz, some pink to red irradiated tourmaline, some apricot morganite and all deep blue to deep green maxixe beryl (may have been irradiated). On rare occasions, some amethyst has been reported to fade very slowly. Any materials colored with an organic dye will normally fall into this slow-fading group. Agates and other porous materials colored with inorganic salts do not fade.
In conclusion: Clearly, the need for fade testing exists. Gem labs should be equipped to supply a fade test result based on a controlled light exposure in the absence of heat. This would be just one more of the “slightly destructive” gemological tests and would be used only when all else fails.
Ethical vendors of jewelry, gemstones and gemstone rough should be willing to have their material tested in this manner and accept any reduction or loss of color that may occur if a rapidly fading color is present. It’s reasonable for the person submitting material for a fade test to sign a disclaimer recognizing that color loss may occur.
Rapidly fading materials can be identified with the test described. Slowly fading materials need one to several weeks of exposure. Materials with a stable color will not be affected at all by such a test. For material where rapidly fading and non-fading forms can coexist, one or two additional 24-hour exposures may be needed to complete the fading and reach the stable color.