GSA 2022 Annual Meeting

Si Athena Chen, Tao Hsu, and James E. Shigley

The Geological Society of America (GSA) held its annual meeting October 9–12 in Denver, Colorado. GIA’s tenth technical session, entitled “Gemological Research in the Twenty-First Century—Gem Materials and Localities,” featured twelve oral presentations and five posters (figure 1), covering broad topics including diamond, sapphire, tourmaline, sunstone, nephrite, and other gem materials.

Colored Gemstones and Other Subjects. The session started with recent research on colored gemstones. Dr. Peter Heaney (Pennsylvania State University, State College) opened with a keynote presentation addressing the structural coloration in colored gemstones that is produced by micro- or nano-structures. This structural coloration results from the interference of nanoparticle inclusions with light waves, including film interference, diffraction grating, scattering, and photonic crystals. Nanoparticles were characterized by a variety of techniques, including scanning electron microscopy (SEM), focused ion beam (FIB), transmission electron microscopy (TEM), and atomic force microscopy (AFM). Dr. Heaney summarized his group’s state-of-the-art research on iridescent gemstones, including three-dimensional photonic arrays of hematite (Fe₂O₃) nanoneedles, quasi-periodic void layers in goethite (FeOOH), blazed gratings of Brazil twinning in iridescent quartz, and hierarchical oscillation layers in iris agate. Continuing with the topic of structural colorization, Dr. Shiyun Jin (GIA, Carlsbad) investigated the cause of special colors (green, red, and watermelon) and the optical effects of pleochroism in Oregon sunstone. The absorption and scattering power of copper nanoparticles in sunstone was tested with experimental absorption/scattering spectra and modeling based on Maxwell’s equations. By comparing his experimental results and simulations, Dr. Jin concluded that the metallic copper nanoparticles in Oregon sunstone absorb and scatter incident light differently with respect to light polarization, thus yielding various color effects. Che Shen (Munsell Color Science Laboratory, Rochester Institute of Technology) studied the color change in sapphire caused by varying the amounts of six major chromophores (V³⁺, Cr³⁺, Fe³⁺, Fe²⁺-Ti⁴⁺, h-Cr³⁺, and h-Fe³⁺). Using chromatic adaptation transformation (CAT), the color of sapphire under daylight and incandescent light were modeled by combining various chromophores. Shen concluded that blue/purple sapphire is caused by Fe²⁺-Ti⁴⁺ and Cr³⁺, green/reddish brown sapphire by Fe²⁺-Ti⁴⁺ and Fe³⁺, and pink/purplish brown sapphire by Fe²⁺-Ti⁴⁺, Cr³⁺, and Fe³⁺.

Other oral presentations focused on sapphire formation. Dr. Aaron Palke (GIA, Carlsbad) investigated the multiple types of melt inclusions in Montana’s secondary sapphire, especially silica-rich felsic, sulfide, and polycrystalline melts (figure 2). The presence of multiple types of melt inclusions indicates that these sapphires are of magmatic origin. Further research on Montana sapphire by Alexander Goodsuhm (GIA, Carlsbad) constrained the formation conditions for Montana sapphire using zircon concentrations in rutile inclusions. Zircon concentrations in rutile inclusions ranged from 194 to 1340 ppm using inductively coupled plasma–mass spectrometry. Missouri River sapphire was suggested to form at a higher temperature (~750°C) than Rock Creek sapphire (~660°C). Dr. Rachelle Turnier (GIA, Carlsbad) studied sapphire from Myanmar, Madagascar, and Sri Lanka, pairing photoluminescence (PL) spectra and trace element contents. Dr. Turnier observed that Cr³⁺ in sapphire is an efficient luminophore with intense red luminescence emission. Chromium in the corundum lattice can overcome high iron concentration and still cause intense luminescence. For example, 3 ppmw Cr³⁺ and 2500 ppmw Fe³⁺ can still yield luminescence of 1,000,000 counts/s/mW.

Other colored gemstone studies included beryl (oral), tourmaline (oral), and nephrite (poster). Dr. Rhiana Henry (Earth, Ocean and Atmospheric Sciences Department, University of British Columbia, Vancouver) investigated structural changes in response to chemical composition variation in beryl (Be₃Al₂Si₆O₁₈). Beryl’s crystal structure was analyzed using single-crystal X-ray diffraction (XRD), and chemical composition was acquired via electron microprobe analysis (EPMA). She concluded that beryl’s crystal structure could be calculated using the average ionic radii of the measured cations at the aluminum site (octahedron site) and the beryllium site (one of the tetrahedron sites). Dr. Yusuke Katsurada (GIA, Tokyo) investigated the dominant chromophores (Fe²⁺ and Cu²⁺) in cuprian Paraíba tourmaline. Oriented wafers were prepared to collect both o-ray and e-ray ultraviolet/visible/near-infrared (UV-Vis-NIR) absorption spectra. Copper and iron concentrations were measured with laser ablation–inductively coupled plasma–mass spectrometry. This study contributes to the simulation of unpolarized UV-Vis-NIR spectrum and the approximation of copper and iron concentrations in cuprian tourmaline. Ping Li (Tongji University, Shanghai) demonstrated nephrite’s microstructure as an indicator of origin. Nephrite microstructure, including grain size and mineral aggregation, were observed under the petrographic microscope. Nephrites from Xiaomeiling, Maxianshan, and Longxi in China underwent different extents of tectonism and metasomatism.

In recent years, GIA’s Research and Development department has broadened its areas of interest, which was demonstrated in two other presentations. Dr. Matthew Hardman (GIA, Carlsbad) investigated the capability of machine learning to classify treated/untreated and synthetic/natural gemstones. More than 3,000 PL spectra of diamonds and 500 sets of trace element data of pearls were studied using the Boruta statistical algorithm. This approach identified PL peaks at 67.6, 503.2, 524.3, and 575.0 nm as important in identifying the treatment of CVD lab-grown diamond. Using machine learning, Dr. Hardman confirmed that manganese concentration is significant in identifying seawater or freshwater origin in pearls. Dr. Sona Taijiryan (GIA, Carlsbad) presented a poster on the main gem trading routes of the early modern period, from 1500 to 1800 CE. By translating an eighteenth-century manuscript from an Armenian gem merchant in India, Dr. Tajiryan demonstrated the East-West trade routes for the most popular gems in the first half of that century, including Indian diamonds, Southeast Asian rubies and spinels, Colombian emeralds, Sri Lankan pearls, and Mediterranean corals.

Diamonds. Dr. Evan Smith (GIA, New York) investigated a 910 ct type IIa diamond from Lesotho to indicate its formation in the mantle. This type of large and highly pure diamond is commonly known as CLIPPIR (Cullinan-like, large, inclusion-poor, pure, irregular) diamond. Inclusions such as cohenite (Fe₃C), troilite (FeS), and metallic Fe-Ni-C-S melt were identified using synchrotron X-ray diffraction at Argonne National Laboratory. The geochemistry of these inclusions confirmed a sublithospheric origin of the host diamond and provided evidence that during the diamond’s formation, subduction of basaltic ocean crust to the lower mantle was occurring. Dr. Mei Yan Lai (GIA, Carlsbad) shared her recent doctoral research completed at the University of Alberta, which investigated lithospheric mantle composition and volatile recycling in the West African Craton. Inclusion mineralogy, major/trace elemental composition, and carbon/nitrogen host isotopic analyses for 105 diamonds from the Koidu mine in Sierra Leone were systematically analyzed. Dr. Lai indicated two significant and distinct episodes of eclogitic diamond formation in Koidu diamonds: cores precipitated from crustal melts and rims formed from mantle-derived melts.

Other presentations examined diamond in sedimentary systems. In alluvial environments, alpha radiation can cause a green or brown surface color. Dr. Christopher M. Breeding (GIA, Carlsbad) reported on his recent study of a rare pink stain in a natural type Ib Fancy brownish yellow diamond, a feature also caused by radiation damage. PL spectroscopy suggested that high concentrations of nitrogen-vacancy (NV^–) defects caused the pink color. It was suspected to have originated as a green diamond that turned pink as a result of higher temperature burial. Also, Dr. Roy Bassoo (GIA, Carlsbad) reported on various thermal annealing experiments on natural colorless diamonds to imitate diamond residence in a natural sedimentary system. Nitrogen content and aggregation, defect concentration, and luminescence color response upon heating were studied using Fourier-transform infrared (FTIR) spectroscopy, PL spectroscopy, and optical UV luminescence, respectively. Dr. Bassoo concluded that diamonds in a sedimentary system do not change their bodycolor, but such a system can influence defect-related luminescence.

GIA poster sessions highlighted three interesting colored diamond projects. First, Sarah Arden, Abadie Ludlam, and Elina Myagkaya (GIA, New York) presented their study on etch channels in a 3.17 ct pink type IIa diamond. A profusion of meandering (“worming”) channels of various lengths embedded in this pink diamond were observed with micro X-ray computed tomography (CT). High strain was observed under the polariscope, and GR1 defects were mapped as clusters around etch channels using PL. The authors hypothesized two possible formation mechanisms for the etch channels’ formation: (1) a type of line defect causing localized strain that is more vulnerable to dissolution and/or (2) Rose channels caused by micro-twinning and subsequently etched by mantle fluids. Skyelar Caplan (GIA, New York) reported a new color center (525 nm) as the cause of type Ia Fancy pink diamond. The 525 nm band together with an H-related 835 nm broad band caused a transmission for yellow and green light (610–660 nm) and resulted in a pink color. The 525 nm band observed in the UV-Vis absorption spectrum was related to high nickel content. Stephanie Persaud (GIA, New York) investigated two types of color-changing diamonds, chameleon and cryogenic, using spectroscopic analysis. Chameleon diamond changes from grayish green to orangy yellow upon heating, while cryogenic diamond changes from dark gray to light yellow when cooled to freezing temperatures. The cause of color change upon heating/cooling was attributed to changes in energy within the electron band gap.

In addition to the research talks and posters, GIA’s booth in the exhibition hall also attracted many visitors. Robert Weldon (GIA, Carlsbad) hosted a demonstration of the use of light and camera settings for gemstone photography. He shared the stories and knowledge behind his photographs of the Hope diamond, the Dom Pedro aquamarine, and other notable gems. Nathan Renfro (GIA, Carlsbad) demonstrated techniques for inclusion photomicrography (figure 3) using different types of light sources.

An evening reception jointly hosted by GIA and the Mineralogical Society of America was held on October 11. The event provided gemologists and mineralogists with a great opportunity to exchange ideas and facilitate collaboration. In another GSA technical session, Dr. James Shigley shared GIA research opportunities with early-career mineralogists and crystallographers. Overall, the 2022 GSA meeting attracted strong attendance and notable interest in the gemology technical sessions and exhibits. We look forward to the 2023 GSA meeting, scheduled for October 15–18 in Pittsburgh, Pennsylvania.