Gem News International Gems & Gemology, Spring 2020, Vol. 56, No. 1

Dyed Chalcedony Imitation of Chrysocolla-in-Chalcedony

Shu-Hong Lin, Yo-Ho Li, Huei-Fen Chen, and Jiann-Neng Fang

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Raman spectra of loose chalcedony, blue chalcedony, and chrysocolla.
 
Figure 1. The Raman spectrum of the loose chalcedony compared to that of chrysocolla and blue chalcedony reveals a mineral composition of quartz (463 cm–1) and moganite (501 cm–1) but a lack of chrysocolla inclusions (3619 cm–1).

Chrysocolla-in-chalcedony, also known as gem silica or blue chalcedony in Taiwan, is the most valuable chalcedony variety on the Taiwanese market. The beautiful greenish blue color is derived from micro-inclusions of chrysocolla, which can be identified by observation of a peak at 3619 cm–1 in the Raman spectrum. This peak can be assigned to OH groups in chrysocolla. Therefore, the color origin for this material is fundamentally rooted in the presence of Cu2+ ions in the structure of the chrysocolla inclusions. In the past few years, a large number of dyed chalcedony imitations have appeared in Taiwan’s market. The blue color of chalcedony dyed by copper salts, and that of natural specimens containing chrysocolla, is caused by Cu2+ ions.

Recently, a parcel of loose chalcedonies was sent to the Taiwan Union Lab of Gem Research (TULAB) for identification. These stones were submitted as natural blue chalcedony, but Raman spectroscopy later confirmed them as chalcedony without the characteristic peaks of chrysocolla (figure 1).

Cross-section of chalcedony cabochon shows blue mantle zoning.
Figure 2. A blue mantle zoning on the cross section of the loose chalcedony is due to the bath of copper dye. Photo by Shu-Hong Lin.

With the owner’s consent, we cut one cabochon and polished the cross section displaying a blue mantle zoning from surface to center parallel to its profile (figure 2). The sample was analyzed with EDXRF, and concentration mapping on the cross section confirmed that copper was concentrated on the surface and decreased toward the interior, which is strong evidence for dyeing with copper salts (figure 3).

Concentration map of copper on the dyed chalcedony.
Figure 3. The concentration mapping for copper on the cross section of dyed chalcedony, which shows higher copper concentration in the periphery and lower in the interior; the different colors on the right represent the degree of relative concentration for copper from high to low.

Twenty pieces of chalcedony dyed with copper salts were further analyzed with EDXRF and compared to results from twenty pieces of natural blue chalcedony. EDXRF results indicated that the Si/Cu ratio of chalcedony dyed with copper salts was much higher than that of natural blue chalcedony (400–600 and 4–50, respectively). The content of Cu was relatively low in dyed chalcedonies tested.

There are many types of dye used for the color enhancement of chalcedony. Although a series of tests like those used above provide a comprehensive comparison between natural blue chalcedony and the dyed chalcedony analyzed in this research, it requires further verification whether these methods can be applied to other dyed chalcedonies.

Shu-Hong Lin, Yo-Ho Li, and Huei-Fen Chen are with the Institute of Earth Sciences at the National Tawain Ocean University, and Jiann-Neng Fang is with the National Taiwan Museum, in Tapei.