AN UPDATE ON EMERALD TREATMENT;THE RELATIONSHIP BETWEEN FISSURE COUNT
AND DEGREE OF TREATMENT IN EMERALDS
By Ron Ringsrud
(This 8-page article is also written in a short one-page summary)
In an analysis of 41 emeralds, a method of determining the nature and extent of fissure filling in emeralds is tested. Gemologists who see emeralds from Colombia have recently been alert for signs of fissure filling by looking for orange or blue flash effects similar to those found in clarity enhanced diamonds. It was thought that these flash effects, when found in emeralds, generally indicate the use of Opticon (a synthetic resin). A survey carried out in the emerald trade in Colombia has revealed that in Colombian emeralds these flash effects come mostly from a synthetic product called palm oil and not from Opticon. Detection and permanence of Cedarwood oil, palm oil and synthetic resins is discussed.
NOTES FROM BOGOTA, COLOMBIA
Just when the international gem trade began to adjust its collective thinking about the new reality of fissure filled diamonds, orange and blue flashes began appearing in Colombian emeralds, giving rise to doubts as to whether the stones have been fissure-filled with Opticon or some other unacceptable resins.
The principle of fissure filling is that while air-filled fissures are highly visible, a fissure filled with transparent oil or some other suitable material will be much less apparent; the stone will look cleaner. In emeralds, which, due both to the nature of the material and to the frequent use of dynamite in mining, fissures and fractures are common. It has been accepted in the international gem trade for the visibility of such fissures to be reduced by traditional oils such as cedarwood oil and Canada balsam.
When orange and blue flashes began appearing in Colombian emeralds the author began studying them in an attempt to understand the changes taking place in the Colombian cutting and treatment centers. The assumption of opticon use, often a forgone conclusion among many, was put in doubt by the total unavailability of Opticon in the trade supply stores in Bogota as well as its total absence on the shelves of the major emerald treatment laboratories. Microscopic study made it apparent that the synthetic resin called palm oil was capable of creating orange (or yellowish orange) and blue flashes in Colombian emeralds. Kammerling (1991) stated that flash effects are not specific to Opticon and could occur from other fillers. Palm oil's use in Bogota as a treatment oil expanded greatly from 1989 onward; just when the orange and blue flashes began to appear in Colombian emeralds. The conclusion of palm oil rather than opticon as the cause of orange and blue flashes in Colombian emeralds was further substantiated by an informal survey of emerald dealers and treaters who do not see opticon in use in Bogota (Jack Rotlevich, Jim Rotlevich, H. Rodriguez, J. Sarmiento, R. Giraldo, O. Lizarazo, L. Lizarazo, Y. Reyes, M. Depotes, G. Angarita, L. Rodriguez, pers. Comm. 1995-6 )
Palm oil (called 'palma' or 'aciete de palma' in Colombia) was investigated in the laboratory of a manufacturer of oils and found to have nothing to do with the palm. It was concluded to be a synthetic resin rather than a vegetable oil and is not miscible with other oils the way that Canada Balsam mixes with Cedarwood oil in order to thicken it. As a non hardening oil, palm oil behaves more like cedarwood oil than like opticon in that it never hardens and can be removed from the stone. Cedarwood remains the preferred oil because, while palm oil may get milky or whitish over time, cedarwood remains colorless and transparent.
The familiarity to the trade of cedarwood oil and the fact that palm oil may dry out over time has moved the Colombian Association of Emerald Brokers and the Colombian Cutters Association to declare palm oil unacceptable. The new guidelines which outlawed palm oil were published in an emerald trade newspaper in Bogota last year (Gema al Dia. 1995). The paper also reported the new CIBJO recommendations from the April 1995 meeting in Europe regarding treatments.
In an interview with Oswaldo Lizarazo, director of Gematrat, an emerald treatment laboratory in the heart of Bogota's emerald district, he stated that although the Association of Emerald Brokers has disallowed palm oil, it still has a demand among some cutters of medium and lightly saturated clean goods with a light tone and good life (called 'cristál in Bogota). It was also reported that the Japanese exporters are also buying Araldit (a technical name for palm oil) in bulk from Colombia (O. Lizarazo pers. Comm. 1996) around the same time that the ICA Gazette reported on "Fluid Epoxy Resins" in emeralds in Japan.
Gematrat is working with the Colombian Office of Technical Norms. This office sets the designation and classification of all industrial products (from food to plastics) and is setting up the norms for the fissure filling of emeralds. The indicated norms for the emerald trade will be cedarwood oil and Canada Balsam or mixtures thereof. Also, there is research being done on a new synthetic filling material.
The main emerald treatment laboratory used to carry out this investigation is the author's own, with apparatus for heating oils to the optimal temperature for penetration into fissures (near 100C.), vessels for creating high vacuums (up to 30 inches of mercury) to remove the air from fissures, and other specialized vessels for pressurizing the heated oil to up to 7000 pounds per square inch. It should be noted that certain oil pressurizing and heating vessels for sale to the trade from Israel and other sources are flawed in that they leave air in the vessel, which, when pressurized, turns the oil into foam which is less than optimally efficient (T. Themelis, J. Rotlevich, Pers. Comm.).
MATERIALS AND METHODS
41 untreated emeralds were viewed in reflected incandescent light at 10x with binocular microscope. A count was made of the surface-reaching fissures on each stone. After fissure-filling, the stones were viewed face up and a percentage estimation was made of the degree of face-up improvement of appearance. An analysis was made to determine the relationship between the number of surface-reaching fissures and the percentage of visual improvement from the fissure-filling.
In order to consistently estimate the degree of face-up improvement of emeralds after treatment 400 emeralds were viewed by the author while in Bogota. Most were in a dry state freshly removed from the acid and alcohol treatment which precedes the oiling while 140 were still on dops after polishing. The acid (a 2:1 mixture of hydrochloric and nitric acids) removes the dark residual polishing powder that accumulates in surface pits, scratches and cracks during the cutting and polishing process. The acid also removes all remaining dop wax. After the acid treatment the stones are boiled in alcohol to remove the acid residue and to prepare the stones for fissure filling.
The nature and extent of fissures was observed and a determination was made as to the probable improvement of the face up appearance of each emerald after treatment. This determination was based simply on the number and size of surface reaching fissures. Then the emeralds were viewed again after treatment and verified as to the extent of face-up improvement. The improvement was measured as a percentage of change in the total face up appearance.
Discussion
Three objectives were pursued in this investigation: to survey dealers, cutters and treaters in Bogotá about fissure filling, to expand on research dealing with fissure filling, and to analyze 41 emeralds for the purpose of creating a method of estimating the degree of enhancement in emeralds. 400 emeralds were also analyzed outside of the laboratory to provide a background for this investigation. The emerald analysis was made to answer the question, "is it possible to estimate clarity enhancements with reasonable accuracy even if the stone is not viewed before treatment?"
Rather than study emeralds from a collection, our sample in Bogotá was taken directly from merchandise on it's way to the wholesale market in 1996. Of those 400 stones, 200 were viewed before treatment in order to verify the visual improvement after fissure-filling and 200 more emeralds were viewed as treated stones first, estimated as to their probable degree of enhancement and then "de-oiled" to verify the estimate and extent of fissures. The emeralds fell into these four categories: negligible improvement, slight, medium and heavy, with negligible being a 0-5% improvement of the overall face-up appearance, slight being 6-20%, medium 21-40%, and heavy being 41% or more.
The conclusion from the 400 stone analysis (which is a good reflection of the market) was that most fissure filled emeralds have a negligible to slight degree of enhancement. The percentages were Negligible: 19.8.1%, Slight: 40.5%, Medium: 30.1%, and Heavy: 9.6%. The only variance was with 'sleepy' stones. They can have fissures that reach the surface but their filling may hardly improve the appearance due to the cloudy nature of the material. This resulted in a tendency to overestimate the degree of enhancement. Almost 20% of all stones tested showed a 'negligible' degree of enhancement. This is a reflection of the good production in the last few years of clean bright emerald rough from the Muzo region.
The laboratory analysis of 41 stones included 30 Colombian emeralds, 5 Zambian and 6 Brazilian emeralds ranging from .61 carats to over 20 ct. A count was made of how many facets were on each stone and how many of those facets have a fissure coming to the surface. An ideal emerald or step cut consists of 41 facets; 17 on the crown and 24 on the pavilion. The facet count on the emeralds in this study was mostly either 41 or 37 (a 37 count was achieved by terminating the pavilion corner facets at the second step rather than the apex of the emerald).
The percentage of facets with fissure openings was then determined. The fissure count included only the fissures that come to the surface; internal fissures will not improve. The method for counting surface-reaching fissures was with 10-15x magnification under reflected incandescent light (Fig. 7 & 8) as detailed in the Detection section.
The degree of clarity enhancement was determined and given a percentage as outlined above. Not surprisingly, there was a direct, one to one relationship between the % of surface-reaching fissures and the % of clarity enhancement. For example, viewing the 37 facets an emerald in reflected light may show 4 with surface-reaching fissures (or 11% of the facets). One could conclude therefore, that the emerald is likely to have about an 11% face up clarity enhancement due to fissure-filling.
Errors in this technique of estimation were usually to the magnitude of up to 5 percentage points (28 out of 41 stones) or up to 10 percentage points (9 out of 41 stones). The tendency was to overestimate the degree of clarity enhancement in sleepy or velvety textured stones and to underestimate the degree of clarity enhancement in lively, crystalline stones. Also, as in diamond grading, the table facet must be weighted more than other facets. A fissure breaking the surface of the table facet from corner to corner or several fissure openings on the table should be counted as 2 or even 4 facets rather than one.
As is evident in most before and after photographs of fissure filled emeralds, the clarity enhancement is often not perfect. The filled fissure can usually still be seen upon careful analysis (Figs. 16, 17, 18, & 20) and the fissure openings always remain, even when a 'sealant' is used. Therefore an estimation of the degree of enhancement can generally be made. One generally reliable indicator for Medium or Heavy enhancement is surface reaching fissures both on the crown and pavilion at the same time. Negligible and Slight enhancement stones often had surface reaching fissures only on the pavilion or on the pavilion and a very small portion of the crown.
It was infrequent to see a dramatically changing emerald. The greatest change seen was 60% while most in the treatment category of Heavy were 40-50%. This is due to the difficulty of obscuring extremely heavy fracturing from view. Also, since surface-reaching fissures are needed to permit the entry of fissure filling material, there is a limit to how many fissures can exist in an emerald before the stone breaks or falls apart. The cutting and polishing process is very traumatic to an emerald and it is a common occurrence that weak stones break during polishing and are recut into smaller, more structurally sound stones. There is no such thing as the fissure-filling material 'holding together' a totally fissured stone.
As will be seen in the discussion that follows, the determination of exactly which oil or resin is in the fissures can be difficult, even using advanced testing procedures. Since emerald treatment is still as much an art as a science it is possible for an emerald to be re-treated as it passes from cutter to broker to wholesaler, each time with a different fissure-filling material and with different techniques. A stone with several materials in the fissures would confuse even the best scientists. However a survey of emerald treaters, dealers and laboratory directors has led this writer to conclude that the question, "How much has an emerald been oiled?" is more important than, "With what?. (Geo. Bosshart, R. Giraldo, O. Lizarazo, H. Hanni, R. Linder, J. Rotlevich, M. Depotes, J. Sarmiento pers. Comm. See box C). It is hoped that, with this study of emeralds from different localities, gemologists who see many emeralds can begin to have greater confidence in estimating degrees of enhancement in emeralds.
FLASH EFFECTS FROM OPTICON AND PALM OIL
A 1.10 carat emerald with a familiar V-shaped fissure that comes to the surface at the table was used as a photo model for the first series of photos. The empty fissure appears in Fig. 1. The emerald was then oiled with cedarwood oil and, although the stone's appearance was quite improved (Fig.2 ), flash effects were absent which is consistent with the properties and understanding of cedarwood oil. A simple and effective way to 'de-oil' or clean an emerald is to use alcohol (acetone is also used) in a heated and stoppered test tube. The stopper creates pressure on the emerald as the alcohol or acetone is heated and further cleans the stone, often discoloring the solvent with the dissolved oil. Once the stone was cleaned and dried, it was treated with palm oil. In Fig. 3 the V-shaped fissure can be seen with three orange flashes near it resulting from the presence of palm oil in the fissure.
Figure 4 shows the same stone but at a different angle and slightly higher magnification with blue flashes. As will be shown, these orange and blue flashes can also be created with the use of Opticon. Figure 4A shows a different palm oil treated stone exhibiting both orange and blue flashes together. After removing the palm oil the same effects were produced in the 1.10 carat stone with Opticon but required slightly different angles of sight and illumination to produce them (Figures 5 and 5A). This investigation shows that both palm oil and Opticon produce orange and blue flashes in Colombian emeralds.
The refractive index (R.I.) of cedarwood oil is 1.515 while the R.I. of the palm oil is closer to that of emerald at 1.570-2. The R.I. of emerald is 1.577 - 1.583 + .002. The flash effect is caused by the slight difference between the fissure filling liquid and the R.I. of the gemstone itself. Opticon produces a similar flash effect as palm oil and has a similar R.I. ( 1.565) as well.
It should be noted that cedarwood oil is not entirely devoid of flash effects although it is generally true. Under very rare conditions, a weak red flash effect can be seen. This, along with points mentioned below underscore the importance of care and experience when diagnosing fissure filling.
DETECTION
Since it is necessary for a fissure to reach the surface to permit the entry of a filler, the finer emeralds with higher clarity generally are less prone to fissure-filling because they have fewer fissures. The more fissures that reach the surface in an emerald, the more opportunity to enhance the appearance of the stone. It was also established that oiling improves only the apparent clarity of the stone; not the color. However if enough fissures obstruct the communication of the stone's body color, then the oiling may improve appreciation of that color rather than improve the color itself. Oiling with colored oil is extremely easy to detect and has not been found in Colombian emeralds (Jack Rotlevich, Jim Rotlevich, H. Rodriguez, J. Sarmiento, R. Giraldo, O. Lizarazo, L. Lizarazo, Y. Reyes, M. Depotes, G. Angarita, pers. Comm. 1995-6 see box C). It is more common in Bogota to see painted emeralds which are sold at street level to unsuspecting tourists who bring them upstairs to the professional offices for verification.
There are stones that exhibit the orange or bluish flash effect even with simple loupe magnification and desk lamp illumination. Most emeralds however, need viewing with a dark-field illuminator microscope at 10 to 30x to detect these effects. A good approach to look first for fissure openings on the surface of the stone by turning the stone under reflected light to see if there are any fissures that come to the surface. A strong incandescent light source is better for finding surface reaching fissures than the fluorescent overhead light source on the microscope. The fissure will be perceived as a thin hairline (Fig. 7). Often a hotpoint touching a suspected fissure or surface irregularity will cause a small amount of oil to expand and come to the surface (Fig. 8). This is one of the most simple and reliable tests although careful analysis is required to further determine how deeply the fissure penetrates the stone. Identifying features inside such as trapped bubbles, subtle outlining of fissure boundaries, flow structures (Kammerling 1991) and dry whitish or milky areas will assist in this determination. Because the fissure is filled, many identifying features are in low relief like changes in transparency and optical distortions.
If treatment is suspected then the stone must be turned in all directions while viewing through the microscope at 10 power or higher. Only when the angle of the fissure approaches the line of sight will the slight difference in refractive indexes cause a orange or blue color to be seen. As Figure 9A reveals, at an oblique angle an inclusion only reveals two innocent looking curved images. As the stone is turned (Fig. 9B), they appear to be related to a larger curve. Then slowly as the angle of the inclusion is at the closest to the line of sight (Fig. 9C), an orange curved line reveals that the two images were really tiny air filled gaps in a large palm oil filled fissure. This fact would have remained hidden unless the stone was turned in every direction.
ERRONEOUS RESULTS
Great care must be taken in order to be certain of the diagnosis of a treated stone. Sometimes an emerald can be concluded to have treatment with palm oil or Opticon on the basis of erroneous observations. The most common erroneous flash effect encountered by gemologists is iridescence caused by empty fissures that are narrower than the width of a wave of light. This will scatter the light into the spectral colors as in Figure 10. The difference between iridescence and a genuine flash effect requires observation of the colors while the stone is slowly turned. The flash effect will be only orange or only blue (or both together) while the iridescence will display several or many colors. Also, iridescence is generally viewed when the fissure is perpendicular to the line of sight and the flash effect is seen when the fissure is almost parallel to the line of sight (see also Gem Trade Lab Notes, Fall '95 'Diamond with iridescence in an incompletely filled fissure' page 198).
Organic stain inclusions or iron stains can cause orangish colors to appear inside an emerald as in Figure 11. Also, it should be noted that the fiber optic illuminator, if turned to less than full power, is yellowish orange and if held near the stone can cause reflections in empty fissures of the same orangish color (Fig. 12). Figure 13 shows clearly how the orangish color of a dark-field illuminated finger can reflect the finger's orangish color into the stone. Care should be taken even with tweezers and stone holders not to let colors reflect into the stone. Figure 14 shows a cabochon emerald reflecting the color blue from the sky due to the microscope being placed close to a window.
PERMANENCE
Guaqueros and miners at the emerald mines in Colombia have been reported as saying that emeralds begin to crack more and more once they were taken out of the ground. Sinkankas (1984) correctly observed that the guaqueros were reacting to the fact that as natural moisture in the fissures begins to dry out, fissures were revealed that were already there. However, new understanding of atomic interactions at the tips of fissures (Mitchalske 1989) reveal more information on the subject. Water molecules (which are .26 nm. in size) are small enough to enter a fissure and absorb into the fissure tip where an oxygen-hydrogen bond in the water and a silicon-oxygen bond in the brittle crystal are both cleaved through a chemical reaction, thus increasing the length of the crack. This is facilitated even more when there is stress on the crystal. Basically, the chemical reaction between the silicon and oxygen reduces the amount of energy needed to make the crack extend.
American Indians used this principle to fissure flint (a quartz mineral) into arrowheads. They would soak or steam the flints before knapping (fracturing) them. In terms of permanence it may be that introducing larger molecules of oil or synthetic resins into the fissure would block the further entry of water molecules (which are abundant in the atmosphere) into the emerald, thus extending the stone's longevity.
Tests of the durability of emerald fissure filling in ultrasonic and cleaning fluid were carried out in 1983 (Ringsrud - Gems & Gemology) and 1991 (Kammerling) which showed that properly treated emeralds stand up well to ultrasonic, needing five minutes or more just to elicit the first visibly discernable changes. Kammerling tested the durability of fissure filling treatments quite thoroughly in a laboratory setting with steam cleaner and ultrasonic (Gems & Gemology, Vol. 27, No.2, pp. 70-85, 1991) and showed Opticon to last slightly longer than the traditional oils.
Rather than repeat these laboratory tests this author examined emeralds treated from up to fifteen years ago. Although they would never care to admit it, most professional gem dealers have a few gems in their inventory that are ten years old or more. With the author's stones and similar goods from other emerald-dealing associates, an analysis of the state of the oil treatment in fourteen stones was carried out.
What emerged from this small sample was that the stones that were from six to fifteen years old were invariably oiled with cedarwood oil (or a mixture of cedarwood and Canada balsam) and, in general, were stable - no drying out or discoloration even though stored in the extremely dry air of floor safes or desk drawers. However, the emeralds faceted and treated two to five years ago had either cedarwood oil or the newer palm oil. About one third of the stones oiled with palm oil showed some signs of drying out - whitish or milky residue in the fissures as well as occasional dendritic patterns of transparency outlined by white residue (see Fig. 6). More proof of the identity of palm oil is seen in Fig. 6A which shows both dried milky residue, indicative of palm oil, along with orange flash effects, which also are indicative of palm oil treatment.
The author's analysis of old jewelry pieces (six pieces form four to eleven years old) corroborate the above stated conclusion that cedarwood oiling is superior to palm oil in over long terms.
As noted by Kammerling et al, even Opticon can be removed somewhat with a steam cleaner or ultrasonic. The author has noted that Opticon treated stones can often be 'de-oiled' by the overnight use of acetone in a warmed and stoppered test tube as outlined above. This may be due to the fact that Opticon was designed for use on carvings and cabochons with pits, cavities and irregularities and not for long narrow fissures in faceted stones. In many cases the hardener may not be able to seal the entire length of a hairline fissure opening, thereby reducing the usefulness of Opticon to that of any other oil as well as reducing the perceived threat to the trade of Opticon - the inability to remove the treatment.
Wholesalers surveyed were asked the question, "How many emeralds per year do you get returned to you because the appearance changed or it 'dried out'-?, ( H. Rodriguez, R. Zajycek, J. Sarmiento, G. Angarita, G. Acuña, G. Ayubi, Pers.comm. See Box C.). The response was usually "about one a year" for the last ten years (all wholesalers surveyed had been in business for more than ten years). One large jewelry manufacturer who sells 30,000 jewelry pieces per year reports an average of ten per year returned because the appearance changed. Then, all were asked, "How many change appearance before you get a chance to sell it? That is, within the first 4 to 8 weeks after returning from Bogotá?" The response was from 5 to 20, indicating clearly that the majority of emeralds change appearance quickly, after which time, they are less prone to dramatic changes. The respondants attributed the problem of changing appearance to the wide use of palma in Bogotá. Because of this, alertness to emerald oiling is more important to the buyer who goes to Colombia than the mainland purchaser. Also, many responsible wholesalers have a policy of not re treating emeralds once they leave Colombia in order to avoid problems with stone's appearance changing after it is sold to the retailers.
The altitude above sea level of Bogota is 8600 feet which means that the oil treatment made at that altitude will be somewhat diminished upon shipping the stones to countries and cities closer to sea level. The varying pressures in the average airplane trip to a city near sea level are enough to accelerate the oil coming out of the stone. If the emerald is not re-oiled again then there can be expected either no change or very little further change in appearance of the stone. This has proved to be the experience of a number of emerald importers in the USA.
Conclusion
With concern over gem treatments very high, any new knowledge or information on the subject serves greatly to reduce the doubt and hesitation that gets in the way of successful buying, selling or appraising. By analyzing emeralds on their way to the market and interviewing dealers, cutters and treaters it is hoped that a greater sense of proportion can be achieved from this study.
Attempts are being made by gemologists in Bogota and the U.S. to distinguish in a treated stone whether the more traditional oils are present or synthetic resins like palm oil and Opticon are used. Although flash effects in an emerald may often indicate synthetic resins it is not a conclusive indicator because of overlapping of the properties of some fillers. Also the R.I. of emerald can be more variable than other gemstones with a range of 1.57 to 1.60 which could at times cause erroneous conclusions.
The differences between the various treatments found in Colombian emeralds are subtle and difficult to analyze. It is more practical to estimate the degree of enhancement than to attempt to determine the exact nature of the treatment inside. There is a close relationship (1:1) between surface reaching fissures and the degree of clarity enhancement in emeralds. Fissure openings on the table as well as the pavilion may indicate heavy treatment while openings only on the pavilion usually indicate normal or minimal treatment.
back to main page . . . . . . . . . back to emerald information
REFERENCES
Bosshart, G. (1991) Emeralds from Colombia, Journal of Gemmology, Vol. 22, No. 7.
Crowningshield R. (1984) Gem trade lab notes: Emerald, oiled. Gems & Gemology Vol. 20, No. 1, pp. 46 47.
Everhart T. (1989) New emerald-treating service to be launched by Yehuda. Rapaport Diamond Report, Vol.12, No.41, p.9.
Farid, Abraham. (1994) Instability Dynamics of Fissure via Parallel Molecular Dynamics. Physical Review Letters 7,3 272. The American Physical Society
Filled emeralds (1991) Jewellery News Asia, No. 80, pp. 88, 90.
Fluid epoxy resin reported in emerald fissures (1991). ICA Gazette, February p. 9.
Fryer C.W (1981) Gem trade lab notes: Emerald substitute, dyed beryl. Gems & Gemology, Vol.17, No.4, pp.227-228.
Gema al Dia. Organo Informativo del Sector Esmeraldifero Colombiano Vol. 1, No. 4, Bogota, Nov. 1994
Gubelin E.J., Koivula T.I (1986) Photoatlas of Inclusions in Gemstones, ABC Edition, Zurich.
Kammerling R.C., Koivula 1.1., Kane R.E. (1990) Gemstone enhancement and its detection in the 1980s. Gems & Gemology, Vol. 26, No. 1, pp. 32-49.
Kammerling R.C., Koivula 1.1., Kane R.E., Maddison, Shigley J., Fritsch E., (1991) Fissure Filling of Emeralds . Gems & Gemology, Vol. 27, No. 2, pp. 70-85.
Kane R.E. (1990) Gem trade lab notes: Emerald, with filled fissures. Gems & Gemology Vol. 26, No. 1, pp. 95-96.
Koivula J.I. Kammerling R.C. (1989) Gem news: Santa Terezinha-emerald treatment. Gems & Gemology Vol. 25, No. 3, pp. 180-181.
Koivula J.I., Kammerling R.C., Fritsch E., Fryer C.W,Hargett D., Kane R.E. i 1989) The characteristics and identification of filled diamonds. Gems & Gemology Vol. 25, No. 2, pp. 68-83.
Liddicoat R.T. (1964) Developments and Highlights at the Gem Trade Lab in Los Angeles. Gems & Gemology, Vol. 11, No. 6, pp. 185-189.
Michalske, Terry and Bunker, Bruce; The Fracturing of Glass, Scientific American June 1989
Mumme I. (1982) The Emerald, Its Occurrence Discnmination and Valuation. Mumme Publications, Port Hacking N.S.W., Australia.
Nassau K. (19841 Gemstone Enhancement. Butterworths, Stoneham, MA.
Ringsrud R. (1983) The oil treatment of emeralds in Bogota, Colombia. Gems & Gemology Vol.19, No.3, pp.149-156.
Sinkankas J. (1981) Emerald and Other Beryls. Chilton, Radnor, PA.
Themelis T., Federman D. (1990) A jeweler's guide to emerald oiling. Modern Jeweler, Vol. 89, No. 5, pp. 64 69.
Webster R. (1983) Gems: Their Sources, Descriptions and Identification, 4th ed. Revised by B. ~ Anderson, Butterworth and Co., London.
Yehuda D. (1990) The Yehuda emerald. AGA Update, December, p. 17.