FEATURE ARTICLES COLOR CHARACTERISTICS OF BLUE TO YELLOW BERYL FROM MULTIPLE ORIGINS Yang Hu and Ren Lu Aquamarine and heliodor are colored by Fe ions, an important coloring agent for beryl. Blue to yellow gem beryl was studied by quantitative spectroscopy and trace-element analytical techniques to explore color char- acteristics and chromophores. Blue color was caused by a 600 nm absorption, while yellow color was attributed to an absorption edge in the violet-blue region. Color ranged from blue to green to yellow due to different pro- portions of Fe ions with various valences and occupancies. Mn content was positively related to Fe, but abundant Mn ions showed no impact on color (unlike Mn in morganite and red beryl). The arrangement of alkali ions and water in channel and the charge compensation mechanism of beryl are discussed. Alkali ions (mainly Na and Cs) and water were localized in the peanut-shaped channels, and all alkali elements (Li, Na, K, Rb, and Cs) were relevant. Though alkali ions and water interacted with transition metal Fe and Mn ions, their influence on blue to yellow color was indirect and rather weak. em beryl is a significant gem species, includ- MATERIALS AND METHODS ing color varieties such as emerald, aquama- Beryls from different origins were gathered and 14 of Grine, heliodor, goshenite, morganite, and red them with various color and alkali content were se- beryl. Blue to yellow beryl has been found in numer- lected for this study (see table 1). They were classified ous locations, including Brazil, South Africa, Russia, in the following color varieties: goshenite (colorless to Ukraine, Canada, Myanmar, the United States, near-colorless), aquamarine (greenish blue to blue), Afghanistan, and China (Belakovskiy et al., 2005). green beryl (green to yellowish green), and heliodor Blue color in aquamarine and yellow color in he- (greenish yellow to yellow). With the exception of two liodor are attributed to abundant Fe ions (Wood and faceted stones and one rough stone, the samples were Nassau, 1968). Fe ions are also present in all other color varieties of beryl, though Fe content is rela- tively low in morganite. Although discussions on the role of Fe ions in blue to yellow beryl are not new, In Brief they have mainly focused on crystal physics and • Color in beryl ranges from blue to green to yellow due chemistry. This article explores the color character- to different proportions of Fe-related absorption. istics and chromophore ions of blue to yellow beryl • The color of green beryl can also come from Cr3+ using quantitative chemical and spectral analysis. and/or V3+ ions. The crystal structure of beryl is unique for having • Alkali elements and water in beryl were found to play a peanut-shaped “channel” along the c-axis, and al- complex roles, but their influence on blue to yellow kali ions in this channel interact with transition color is indirect and likely weak. metal ions. Therefore, we will discuss the features of alkali elements and their roles in beryl color, based on analysis of the channel mechanism. This research was part of a series of ongoing studies on the color fabricated as optical wafers perpendicular (PK-7 and characteristics of beryl. PK-8) or parallel (PK-5, PK-10, PK-9, RUS-8, MOZ-2, BM-1, MOZ-1, AF-3, and AF-2) to the c-axis (figure 1). All samples were investigated by standard gemo- logical testing, Raman spectroscopy, Fourier-trans- See end of article for About the Authors and Acknowledgments. form near-infrared (FT-NIR) and ultraviolet/visible/ GEMS & GEMOLOGY, Vol. 56, No. 1, pp. 54–65, http://dx.doi.org/10.5741/GEMS.56.1.54 near-infrared (UV-Vis-NIR) spectroscopy, and laser ab- © 2020 Gemological Institute of America lation–inductively coupled plasma–mass spectrome- 54 COLOR CHARACTERISTICS OF BLUE TO YELLOW BERYL GEMS & GEMOLOGY SPRING 2020 PK-7 PK-5 PK-10 PK-8 BRA-3 PK-9 BM-1 MOZ-2 RUS-8 AF-3 AF-2 UK-11 UK-10 MOZ-1 Figure 1. The 14 studied beryl samples (0.63–3.50 ct) from various geographic origins. Color ranged from blue to yellow, as well as near-colorless and colorless. Photo by Yang Hu. try (LA-ICP-MS) chemical analyses. Inclusions were rate. Each analysis incorporated a background acqui- captured using a Leica M205A microscopic system sition time of approximately 20–30 seconds followed with oblique fiber-optic illumination. Raman spectra by 50 seconds of ablation. A multi-standard quanti- were collected by a Bruker Senterra R200 spectrome- tative calculation method was adopted, with Al cho- ter coupled with a 532 nm laser for identifying various sen as the normalizing element. Calibration inclusions. The resolution was set at 5 cm–1 with a 20 standards of NIST 610, BCR-2G, BHVO-2G, and BIR- second integration time, 2 accumulations, and 20 mV 1G were used as external references (Liu et al., 2008). laser energy. To explore the characteristics of water in Three laser spots for each sample were applied in an the beryl, FT-NIR was performed using a Bruker V80 area that was typically clean with an even color dis- FTIR spectrometer at 2 cm–1 resolution and 32 accu- tribution. UV-Vis-NIR spectra were collected in the mulations. To study the color features, UV-Vis-NIR same area analyzed by LA-ICP-MS. spectra were recorded with a PerkinElmer 650s spec- trophotometer equipped with a 150 nm integrating RESULTS sphere accessory at 1 nm resolution. Gemological Properties. The beryl samples had a re- Chemical analysis was performed by LA-ICP-MS fractive index range of ne=1.568–1.579 and using an Agilent 7500a and 7900 ICP-MS instrument no=1.573–1.586, with birefringence between 0.005 and combined with a GeoLas 193 nm laser. The carrier 0.006, with the exception of samples BM-1 and BRA- gas used in the laser ablation unit was He with a flow 3. Pale green MOZ-1 had the lowest RI (1.568–1.573) rate set at approximately 650 mL/min. Laser ablation among all the samples. Burmese deep blue sample conditions consisted of a 44 μm diameter laser spot BM-1 and Brazilian dark greenish blue BRA-3 had the size, a fluence of 5–6 J/cm2, and a 6–8 Hz repetition highest RI (1.589–1.600) and birefringence (0.009– COLOR CHARACTERISTICS OF BLUE TO YELLOW BERYL GEMS & GEMOLOGY SPRING 2020 55 AB Figure 2. Two-phase in- clusions in blue to yel- low beryl samples. A: Fingerprint-like two- phase inclusions along a a healed fissure plane in aquamarine. B: Hexagonal two-phase 100 μm b 100 μm inclusions viewed down the c-axis in CD aquamarine, consistent with the crystallo- graphic symmetry. C and D: Isolated elon- gated rod-like two- phase inclusions parallel to the c-axis in heliodor. Photomicro- c c graphs by Yang Hu. a a 100 μm 100 μm 0.010). All the beryl samples displayed weak to mod- Microscopic Observation. Two-phase inclusions were erate dichroism. All bluish beryl samples showed the most common type in the beryl samples, contain- more blue color along the e-ray than the o-ray. It ing one gas bubble (CO2) floating in at least one kind should be noted that the dark hue of sample BRA-3 of fluid (figure 2). Minor CH4, H2S, and N2 were found was caused by extremely tiny cloudy dark inclusions only in some of the Pakistani beryl samples. Normally throughout nearly the whole sample. The lemon and the Pakistani samples had two liquid phases (CO2 and yellow heliodor samples showed weak dichroism but water with minor dissolved CO2), while samples from a relatively saturated color. other deposits hosted one liquid phase (water). Some- TABLE 1. Blue to yellow beryl samples selected for this study. Sample no. Variety Geographic origin Color PK-7 Goshenite Shigar Valley, Pakistan Near-colorless PK-5 Goshenite Shigar Valley, Pakistan Near-colorless PK-10 Goshenite Shigar Valley, Pakistan Colorless PK-8 Aquamarine Shigar Valley, Pakistan Near-colorless PK-9 Aquamarine Shigar Valley, Pakistan Pale greenish blue BRA-3 Aquamarine Brazil Dark greenish blue RUS-8 Aquamarine Russia Greenish blue MOZ-2 Aquamarine Mozambique Blue BM-1 Aquamarine Mogok, Myanmar Deep blue MOZ-1 Green beryl Mozambique Pale green AF-3 Green beryl Africa Yellowish green AF-2 Heliodor Africa Greenish yellow UK-11 Heliodor Unknown Lemon yellow UK-10 Heliodor Unknown Golden 56 COLOR CHARACTERISTICS OF BLUE TO YELLOW BERYL GEMS & GEMOLOGY SPRING 2020 RAMAN SPECTRA Figure 3. This multi- 223 (S) fluid II phase inclusion had two fluid phases, one S O gas gaseous phase, and one crystal O 474 (S) crystal phase. Abbrevi- O 686 (B) fluid I 3606 (H1) ations: B—beryl, S— 1388 (C1) 100 μm sulfur, O—orpiment, 1243 (C2) C1—CO2 in the inclu- crystal: sulfur+orpiment sion, C2—CO in the 1284 (C1) 3200–3500 (H2) 2 fluid I: water beryl structure channel, INTENSITY (a.u.) H1—water in the beryl 2913 (CH ) gas: CO2+CH4 4 structure channel, and fluid II: CO2 H2—water in the inclu- B B B sion. Spectra are offset B B B B aquamarine (E⊥c) vertically for clarity. Photomicrograph by 0 500 1000 1500 2000 2500 3000 3500 Yang Hu. RAMAN SHIFT (cm–1) times a mineral phase was hosted in the fluid, such as disappeared after heating above 400°C. Raman analy- native sulfur and orpiment in Pakistani samples (see sis of this two-phase inclusion detected no gas or fluid.