As one of the five most prestigious gemstones, ruby has won broad market favor for its unique allure and exceptional value. Heat treatment is the most common method for ruby and has long been widely used and accepted in the market, as heating can improve a ruby's color.
Recently, however, gemologists at GUILD Gem Laboratories found obvious external substances within the fissures of multiple rubies during testing. Under the gemological microscope, these substances appeared highly similar to the fillers seen in clarity-enhanced emeralds.
Further confirmation through infrared spectroscopy, Raman spectroscopy, X-ray microradiography, and extensive data collection demonstrated that this group of rubies had undergone clarity enhancement.
As is well known, due to their inherent properties and growth environments, different varieties of colored gemstones develop fissures to different degrees. Emeralds, for example, are generally more prone to fissuring and often display pronounced internal features.
As a result, most emeralds on the market undergo clarity enhancement to improve their clarity. Aquamarine, by contrast, typically has very high clarity and does not usually develop fissures, so clarity enhancement is rarely performed.
Clarity enhancement is a process in which various substances such as oil, wax, or resin are introduced into a gemstone (not limited to oil, though in this article the term “oil” treatment is used as shorthand and is synonymous with clarity enhancement) to improve its appearance. This method can conceal fissures to a certain extent, thereby enhancing the gemstone's overall visual appeal.
Why, exactly, are rubies treated with “oil”? What is the difference between rubies with or without clarity enhancement? Are clarity-enhanced rubies the same in nature as lead-glass-filled rubies? How can they be identified? These are all common questions in the market, and this article addresses them one by one.
“Oil” Treatment vs. No Clarity Enhancement
Rubies that require clarity enhancement usually have well-developed fissures. “Oil” treatment can improve clarity and color to a certain extent. However, the degree of visual improvement largely depends on the number of open fissures in the ruby. In general, the closer the refractive indices of two media are, the less visible their boundary becomes. For example, the refractive indices of ice and water are much closer to each other than those of ice and air, which is why ice can appear nearly invisible in water.
Ruby has a refractive index of 1.76-1.77, while organic substances usually have lower refractive indices, most of them below 1.6. Although this is closer to ruby than the refractive index of air (approximately 1), there is still a difference. Therefore, “oil” treatment can produce a certain clarity-improving effect in ruby, but it is difficult for it to bring about a fundamental transformation. In appearance, clarity-enhanced rubies generally display relatively low transparency.
“Oil” Treatment vs. Lead Glass Filling
Although both involve introducing foreign substances into ruby fissures, “oil” treatment and lead-glass filling are fundamentally different. In terms of starting material, rubies used for “oil” treatment may contain fissures, but their overall structural integrity remains relatively good.
Lead-glass-filled rubies, by contrast, typically contain numerous fissures, and if the internal lead glass were removed, the stone might even break into several pieces.
In terms of appearance, “oil” treatment provides only limited visual improvement, whereas lead-glass-filled rubies can be highly deceptive in appearance. One way to understand this is that “oil” treatment is comparable to cosmetic makeup, while lead-glass filling is closer to the effect of “plastic surgery.”
It is also important to note that clarity-enhanced rubies generally do not require heat treatment, whereas lead-glass-filled rubies must be heated so that the glass becomes fluid enough to penetrate the ruby.
In terms of value, rubies without clarity enhancement are generally worth more than rubies that have undergone clarity enhancement, while clarity-enhanced rubies are worth far more than lead-glass-filled rubies. Lead-glass-filled rubies have existed for a long time and have been extensively studied. In contrast, very little information has been disclosed about clarity-enhanced rubies.
GUILD Gem Laboratories has already conducted broad and in-depth research on the degree of clarity enhancement in emeralds and the types of organic fillers involved. Accordingly, GUILD gemologists drew on the mature research methodology developed for emerald clarity enhancement to carry out an in-depth study of clarity-enhanced rubies.
Gemological Microscopic Observation
In clarity-enhanced rubies, dried filler residues can usually be seen in fissures. Under the gemological microscope, they often appear as relatively thick droplet-like or irregular features, and bubbles may sometimes be present within the filler. In addition, filler residues remaining in surface-reaching fissures can show a difference in luster from the ruby itself. Residual filler in ruby fissures is an important clue in identifying whether a ruby has undergone clarity enhancement.
Dried organic residue in a clarity-enhanced ruby
Infrared and Raman Spectroscopy
The organic substances introduced by clarity enhancement, such as oils and resins, produce characteristic absorption peaks in Raman spectra. Based on GUILD Gem Laboratories' extensive and well-developed research on organic fillers in emeralds, we found that the infrared and Raman spectra of these rubies also display characteristic peaks similar to those of organic fillers found in emeralds. The presence of these characteristic peaks indicates that the rubies have undergone clarity enhancement.
In addition, during routine examination, GUILD gemologists observed another uncommon filler in which the material within the fissures appeared extremely thick under magnification. Because the infrared characteristic peaks caused by minerals such as diaspore and kaolinite within these rubies were relatively prominent, they obscured the characteristic peaks of the filler itself. The gemologists therefore further localized filler-bearing fissures in the clarity-enhanced rubies and performed Raman testing at those positions. Two groups of Raman shifts were observed at 1444 cm-1, 1605 cm-1, and 1652 cm-1, and at 2878 cm-1, 2928 cm-1, and 3061 cm-1. The first three strong bands are all related to phenyl groups. Of the latter three strong bands, the one at 3061 cm-1 is caused by C-H stretching vibration of the benzene ring, while the bands at 2878 cm-1 and 2928 cm-1 are caused by stretching vibrations of methylene (-CH2-) and methyl (-CH3-) groups. This confirms that the filler in this oil-treated ruby is an aromatic hydrocarbon compound (resin).
X-ray Microradiography
X-ray radiation is a high-energy form of radiation capable of penetrating matter and revealing internal structure. X-ray examination operates on the same principle as chest CT imaging in medical examinations and is also similar to the screening devices used in subways, airports, and train stations. However, the X-ray instruments used for gemstones have far higher resolution than those security screening devices. Security scanners generally require only 1 mm or 2 mm precision to perform their function, whereas gemstone-testing instruments require precision roughly three orders of magnitude finer.
X-ray examination is essentially used to determine whether the internal structure of a gemstone is homogeneous. The principle is that materials of different densities will produce different luminous images under X-ray exposure. When a gemstone is internally uniform, the resulting image is also uniform, as in natural unheated or heated rubies. When other substances have been introduced into the gemstone, however, the image will indicate their presence. A classic example is the lead-glass-filled ruby, whose image shows highly conspicuous bright masses caused by differences in internal material density.
In the identification of clarity-enhanced rubies, X-ray microradiography can likewise be used to detect fillers within the gemstone. The organic filler introduced by clarity enhancement is usually present in fissures and appears in X-ray images as darker bands distinct from the body of the ruby.
A comparison of the images above shows that the overall structural compactness of clarity-enhanced rubies is basically consistent with that of unheated and heated rubies. As a result, they are durable and far superior to the structurally loose lead-glass-filled rubies. Clarity-enhanced rubies therefore should not be confused with lead-glass-filled rubies.
Conclusion
In the past, consumers in the Chinese market were often alarmed by terms such as “treatment,” “enhancement,” and “filling.” In recent years, however, as public awareness has increased, the number of professional and trustworthy merchants has grown, testing technology has advanced, and the market has gradually moved to a higher level. Public attitudes toward gemstone enhancement have become increasingly rational, and heated ruby and sapphire as well as clarity-enhanced emerald are now widely accepted in the market.
Clarity enhancement improves a gemstone's overall visual appearance by introducing organic fillers into its fissures. From the supply side, gemstones are scarce natural resources, and the quality of natural production is highly uncertain; therefore, enhancement is an indispensable method. In fact, enhancement is a key step in improving gemstone quality. From the demand side, consumers who want to buy a gemstone that offers commensurate value need to fully understand its various characteristics so they can make purchasing decisions based on their budget and intended use. In the end, the choice depends on individual needs and means.
One principle of gemstone enhancement testing is to ensure that the enhancement is harmless to the human body; another is to fully inform consumers and protect their right to know. As an independent third-party laboratory, GUILD also has a responsibility to guide the market in a scientific, rational, and healthy direction. For this reason, public disclosure of clarity enhancement in ruby is highly necessary. Such disclosure can promote the healthy development of the market, protect consumer rights, and strengthen the sustainable development of the industry.
The identification of clarity-enhanced ruby requires a comprehensive judgment by a professional gemological laboratory using multiple testing methods. By observing organic residues under the gemological microscope, analyzing characteristic peaks in infrared and Raman spectra, examining luminescence phenomena, and using X-ray imaging for internal observation, it is possible to accurately determine whether a ruby has undergone clarity enhancement. Therefore, when purchasing a ruby, a report issued by an authoritative gemological laboratory is of critical importance.
