Lab Notes Gems & Gemology, Summer 2020, Vol. 56, No. 2

Quench-Crackled Dyed Blue Chalcedony Resembling Larimar

Makoto Miura

Download PDF
Two quench-crackled dyed chalcedony round beads resemble Larimar in appearance.
Figure 1. Two of five round beads of quench-crackled dyed chalcedony and a pear-shaped Larimar cabochon. The bottom bead is 10.25 mm in diameter. Photo by Shunsuke Nagal.
Quench-crackled dyed chalcedony showing fractures (left) and curved banding (center) compared with Larimar (right).
 
 
 
Figure 2. A: A quench-crackled dyed chalcedony bead containing whitish zones following fractures; field of view 11.80 mm. B: Another bead showing a curved banding structure; field of view 12.80 mm. C: A pear-shaped Larimar cabochon (also shown in figure 1); field of view 16.20 mm. Note that it has a mottled blue and white color and contains other mineral inclusions. Photomicrographs by Makoto Miura.

Recently, GIA’s Tokyo laboratory examined five round beads that were semitransparent to opaque and showed blue to greenish blue bodycolors with a network-like structure of whitish zones (figures 1 and 2). These beads were acquired at an ornamental gem material shop in Tokyo and were sold as “sea-blue chalcedony,” but the resemblance to Larimar led to our investigation.

Raman spectra comparing the quench-crackled dyed chalcedony with the known Larimar reference.
 
 
 
Figure 3. Raman spectra of quench-crackled dyed blue chalcedony (red trace) and Larimar (blue trace). The dyed chalcedony Raman peaks match those of chalcedony and not Larimar.

The beads were identified as chalcedony based on spot RI readings of 1.53 to 1.54, SG values ranging from 2.48 to 2.65, and microscopic features such as parallel curved bands (figure 2B). Raman spectroscopic features (figure 3) also suggested that the beads were mainly composed of quartz crystals and matched chalcedony (e.g., D. Pop et al., “Raman spectroscopy on gem-quality microcrystalline and amorphous silica varieties from Romania,” Studia Universitatis Babeș-Bolyai, Geologia, Vol. 49, 2004, pp. 41–52). Raman spectra of the beads showed no characteristics of chrysocolla (Spring 2020 Gem News International, pp. 188–189). Qualitative EDXRF analysis detected peaks related to Si, Fe, and Cu, indicating that Cu was likely the color-causing element. Ultraviolet-visible (UV-Vis) spectra also showed a broad band from 500 to 1000 nm that corresponded with Cu. There was no polymer detected by Fourier-transform infrared (FTIR) spectroscopy and no reaction when touched with a hot point. Under a gemological microscope, the stones revealed whitish zones following fractures. In order to consider the overall structure of such fractures, we cut them in half. Cross sections of the beads showed that the fractures with whitish zones were concentrated on the surface (figure 4). Such a structure is a known characteristic of quench-crackled dyed chalcedony and was reported previously (see Winter 2009 Lab Notes, p. 288). These beads probably represent a new color variety.

Quench-crackled dyed chalcedony cross sections showing internal fractures.
 
 
 
Figure 4. Cross sections of two round beads of quench-crackled dyed chalcedony, 10.25 mm in diameter. Note that the white color is concentrated on the surface and along internal fractures. Photos by Shunsuke Nagal.

These quench-crackled dyed chalcedonies resemble Larimar in color and appearance. Larimar is a rare blue variety of pectolite with the ideal chemical formula of NaCa2Si3O8(OH) that displays white, green, and pale to sky blue colors. Blue color seen in Larimar is believed to be caused by the presence of small amounts of Cu2+ within its structure (e.g., R.E. Woodruff and E. Fritsch, “Blue pectolite from the Dominican Republic,” Winter 1989 G&G, pp. 216–225). We compared these beads with Larimar (the pear cabochon on the right in figure 1) to reveal their differences. The Larimar cabochon showed a mottled blue and white color with network-like whitish zones and other natural mineral inclusions (figure 2C) (again, see Woodruff and Fritsch, 1989). It had a spot RI reading of 1.60 and an SG of around 2.80. On the other hand, the quench-crackled dyed blue chalcedony beads had whitish zones following a network of fractures (figure 2B) and no other mineral inclusions.

This material can easily be separated from Larimar using standard gemological testing such as RI and SG. Advanced testing is useful in confirming the cause of blue color and the presence of polymer treatment.

Makoto Miura is staff gemologist at GIA in Tokyo.