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Article Index- Gemstone Enhancement and its Detection in the 2000s: Dyeing and Bleaching
Gemstone Enhancement and its Detection in the 2000s:
Dyeing and Bleaching
by Shane F. McClure, Robert E. Kane, and Nicholas Sturman
from GEMS & GEMOLOGY, Vol. 46, No. 3, pp. 218–240.
© 2010 Gemological Institute of America, Fall 2010
Published on JTV.com: January 2012
This is Part III of a four part series on gemstone enhancement. See Part I and Part II.
See the end of this downloadable PDF for About the Authors, Acknowledgments, and a complete list of References.
Advances in technology and increased demand for lower-priced gemstone materials contributed to the proliferation of new treatments throughout the first decade of the 2000s. The developments that made the most difference were the diffusion treatment of corundum with beryllium, diffusion of copper into feldspar, clarity enhancement of ruby and diamond, and heat treatment of diamond, ruby, and sapphire. Gemological laboratories and researchers have done their best to keep up with these treatments, and the jewelry trade has struggled with how to disclose them. This article summarizes these developments and the methods used to identify the various enhancements.
DYEING
Although it dates back to the time of Pliny (23–79 AD), dyeing continues to be seen in nearly every gem material that is porous or has surface-reaching fractures. Careful microscopic examination will frequently reveal the presence of dye in cracks and around grain boundaries. In a number of porous materials, rubbing the surface with a cotton swab soaked in acetone or a 10% hydrochloric acid solution can identify the presence of dye. In others, absorption spectra can provide proof of dyeing.
Pearls.
Dye continues to be used to improve the appearance of lower-quality natural and cultured Wentzell, 2005). While the majority of dyed pearls are nacreous, dye may also be used to make nonnacreous pearl imitations more convincing, such as those mimicking Melo pearls (Wentzell, 2006). Of ongoing concern since the late 1990s is the detection of dyed “golden” cultured pearls (figure 23; “Concerns raised. . . ,” 2003; Liu and Liping, 2007). Some samples present identification challenges, requiring the use of chemical analysis to detect trace elements such as iodine. Other developments involve the use of additional whitening compounds in freshwater non-beaded cultured pearls (Shouguo and Lingyun, 2001) and the use of metallic dyes injected into pearl sacs (“Pre-harvest colour-treated Akoya unveiled,” 2008; Coeroli, 2010). A form of dyeing marketed as “lasering” has also been reported. This is said to produce dark “peacock” green or dark purple colors (Liping, 2002).
Other Gem Materials.
Several other dyed gem materials were encountered during the decade. Blue and green diamond crystals were found to owe their color to dyeing (Van der Bogert, 2005). Quartzite was dyed red to imitate ruby (Mayerson, 2003a), whereas green dye was found in quartzite to resemble emerald (Milisenda, 2003). Mayerson (2003b) described an effective simulant for high-quality jadeite: a tricolored (lavender, green, and orange) dyed and polymerimpregnated quartzite bangle bracelet. Tan et al. (2006) used light-induced autofluorescence spectroscopy to identify dyed polymer-impregnated jadeite. Of particular interest was dyed jadeite found to resemble nephrite jade (Mayerson, 2004).
Low-quality red and blue corundum were found to have been dyed (Milisenda, 2004). A parcel of faceted “rubies” purchased in Afghanistan was identified by Milisenda (2005b) as dyed sillimanite. Dyed blue carbonate minerals, such as magnesite and dolomite, were sold as turquoise (“Some dyed minerals. . . ,” 2000). To imitate common opal from the Peruvian Andes, marble was dyed pink and fashioned into beads (Milisenda, 2006). Raman and IR spectra identified dyed black chalcedony in an attractive pendant set with diamonds and pearls (DeGhionno and Owens, 2003). A copper-based dye was detected with UV-Vis-NIR spectroscopy in a natural-appearing chalcedony bead (Inns, 2007a).
BLEACHING
Bleaching is a process that uses agents such as acids or hydrogen peroxide to remove unwanted color from a gem. Only a limited number of materials will respond to such treatment.
Jadeite.
Jadeite with brown staining caused by natural iron compounds is often bleached with acid. This treatment started in the 1990s and was categorized in the impregnation section of McClure and Smith (2000). This is because jadeite treated in this manner must be impregnated with polymers, as the acid damages the structure, making it very susceptible to breakage.
This treatment has become commonplace in the jadeite market. However, the bleaching itself typically cannot be detected, only the polymers used for impregnation (Sun, 2001; Fan et al., 2007). The treatment is now being used on nephrite jade as well (Jianjun, 2005).
Pearls.
Bleaching is considered an “acceptable” pearl treatment due to the difficulty of proving a pearl’s exposure to chemicals such as hydrogen peroxide. All types of pearls are routinely bleached: natural, bead cultured, and non-bead cultured. Akoya cultured pearls continue to be routinely bleached and “pinked” (Roskin, 2002b). Bleaching is also known to be a major component of the proprietary process used to produce the “chocolate” cultured pearls (figure 24) that entered the market during the decade (Zachovay, 2005; Hänni, 2006b; Wang et al., 2006c; Federman, 2007c).
Other Materials.
While there are undoubtedly additional porous materials that could be bleached, the only other one we could find reference to is coral. Black coral is bleached to “golden” coral, which is easily identified by its distinctive structure (Weldon, 2003).
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