Lab Notes Gems & Gemology, Spring 2018, Vol. 54, No. 1

Unusual Orange Pyrope-Spessartine-Grossular Garnet

Orange pear-shaped pyrope-spessartine-grossular garnet.
Figure 1. The unique composition of this 7.97 ct orange pear-shaped garnet was revealed as pyrope-spessartine-grossular. Photo by Robison McMurtry.

GIA’s New York laboratory recently examined an orange pear-shaped faceted stone (figure 1). Standard gemological testing revealed a refractive index of 1.771 and a hydrostatic specific gravity of 3.89. The fluorescence reaction was inert to long-wave and short-wave UV light. The stone did not show any pleochroism when viewed with a dichroscope. Using a handheld spectroscope, absorption bands located at 410 and 430 nm in the blue and violet sections were clearly observed. All of these gemological properties are consistent with pyrope-spessartine garnet based on the classification system from Stockton and Manson (“A proposed new classification for gem-quality garnets,” Winter 1985 G&G, pp. 205–218).

Besides standard gemological testing, all garnets submitted to GIA laboratories are routinely analyzed by energy-dispersive X-ray fluorescence (EDXRF) spectroscopy (see column 4 of the supplementary table below). EDXRF results showed that the stone was predominantly composed of 15.37 mol.% pyrope, 53.70 mol.% spessartine, and—surprisingly—25.39 mol.% grossular. Unlike normal pyrope-spessartine garnet submitted to GIA that contains a small amount of grossular (less than 10 mol.%), this garnet had a much higher grossular component that had never been reported before in gem-quality garnets, to our knowledge. (Note that “malaia/malaya” is a trade name for yellowish, reddish, or pinkish orange pyrope-spessartine garnets that can potentially contain 2–94% spessartine, 0–83% pyrope, 2–78% almandine, 0–24% grossular, and 0–4% andradite. The garnet we examined falls into this concentration range.)

To validate the accuracy of the chemistry, we performed laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analysis using a Thermo Fisher iCAP Qc ICP-MS coupled with a New Wave Research UP-213 laser ablation unit to obtain an additional set of chemical composition. USGS glass standard GSD-1G and GSE-1G were used as external standards. 29Si was used as internal standard. The LA-ICP-MS results showed good agreement with the EDXRF results (see columns 1–3 of the supplementary table above).

(Pyrope + grossular)-spessartine-almandine ternary diagram.
Figure 2. (Pyrope + grossular)-spessartine-almandine ternary diagram with data points based on molecular percentages for pyralspite garnet (modified after Stockton and Manson, 1985). The orange spot is the average of three sets of LA-ICP-MS data (table 1, columns 1-3). Refractive index and spectral boundaries are used to distinguish among the six proposed garnet species labeled in the diagram.

Garnets are a group of isometric nesosilicates with the general chemical formula X3Y2Z3O12. Natural rock-forming silicate garnets are commonly divided into the pyralspite (pyrope, almandine, and spessartine) and ugrandite (uvarovite, grossular, and andradite) groups. In pyralspite, Al3+ occupies the Y-site and the X-site may contain Mg2+, Fe2+, or Mn2+; these garnets are composed predominantly of pyrope (Mg3Al2Si3O12), almandine (Fe2+3Al2Si3O12), and spessartine (Mn3Al2Si3O12) end members. Stockton and Manson (1985) presented a ternary plot (figure 2) to gemologically classify the pyralspite garnet species, and this showed the correlation among refractive index, visible spectroscopic observation, and chemistry. They determined that pyrope-spessartine should have a refractive index between 1.740 and 1.780 along with clear 410 and 430 nm absorption bands. We concluded that the orange pear-shaped garnet should be classified gemologically as pyrope-spessartine garnet.

To understand why this garnet with a high grossular component still shows the same gemological properties of normal pyrope-spessartine, we plotted the orange garnet chemistry in this ternary plot by combining pyrope with a grossular component. The orange spot representing the orange garnet appeared in the pyrope-spessartine region, because the refractive index of pure pyrope (1.714) is close to and functions almost the same as the RI of pure grossular (1.734) in contributing to the combined refractive index. Since there is not a specific gemological species name for this orange garnet with higher grossular than pyrope component, the authors suggest that the stone be called pyrope-spessartine-grossular. This case demonstrates that sophisticated chemical analysis of garnet should be practiced routinely in gemological laboratories to make sure unique garnets are not misidentified.

Pamela Cevallos is a research stone collection gemologist at GIA in New York. Ziyin Sun is a staff gemologist at GIA in Carlsbad, California.