“Gota de aceite” Effect in Emerald: Observation and Cause
Abstract: Emeralds with highly saturated green color and pure clarity are strongly cherished and coveted by gem dealers and collectors. Those green beauties with the so called “Gota de aceite” optical phenomenon are rarer. Rarity of such samples causes the scarcity of related research on this subject. This article reports on the gemological features of one 5.1ct emerald with distinct “gota de aceite” effect. Also, the possible cause of “gota de aceite” is described and illustrated in crystalline structural perspective.
Key words: emerald; “gota de aceite”; color
In December 2016, GUILD Gem Lab received a green gemstone ring for testing. The submitted ring contains an octagonal shape faceted green stone, weighing 5.1ct indicated by the hallmark, with fancy cut diamonds around as shown in Fig. 1.
The standard gemological properties of the green stone were consistent with those of emerald. This stone exhibits vivid green color under tungsten lamp and strong red color were seen through Chelsea color filter (CCF) as shown in Fig. 2. The distinct contraction raised our doubts, as it is known that some synthetic emerald may exhibit strong red color under CCF. A quick glance at this emerald under-microscope reinforced our hypothesis. Viewing perpendicular to the table, “zigzag-like” or “water ripple-like” structures, which were similar to those growth structures in hydrothermal synthetic emerald, were easily observed, as shown in Fig. 3. Similar but not the same.
 Fig. 3. “Water ripple-like” structures |  |  |
In addition, there were little inclusion within this stone except for several small fractures near the girdle which were not seen by naked eyes, and high clarity caused more doubts in our mind. Suspicious as we were and more precisely speaking we were almost 90% sure that this stone was synthetic, however, we still have to do more careful and meticulous observation and testing for a 100% sure, following The GUILD Standard Testing Procedure.
1 Further observation and testing
In spite of the limitation of observing directions and conditions, for that this emerald was mounted in a ring, we still found something interesting when viewing near the girdle along the length of the stone, or mesmerizing to be precise. As shown in Fig. 4, “porphyritic” or “granulous” structures were seen under high magnification 90X. Several spots were apparent with oily appearance.
A further observation illustrated that the “spots” exhibit regular outlines and green color with different intensity. As shown in Fig. 5~6, these two spot showed distinct hexagonal outlines, which is in good agreement with the crystal habit of beryl, who belongs to hexagonal crystal system. Step-like growth structure were also observed, revealing the growing procedure of this sample.
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Fig. 4(b). Area A (left) and area B (right) show distinct hexagonal outlines with saturated green color
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| Fig. 5. Emerald crystalline structure viewing along c-axis | Fig. 6. Emerald crystalline structure combined by six units |
Further testing of FTIR spectroscopy and comparison with GUILD IR spectra database confirmed that this emerald was natural, as shown in Fig. 5. Chemical compositions of major and trace elements analyzed by EDXRF with more than 330 spots applied were in good agreement with those of Colombian based on abundant data accumulated over three decades by Mr. Charles Carmona.
2 Analysis
The combination of the whole scenes, including the high clarity and oily appearance, were in very good agreement with the legendary term circulating among experienced emerald dealers and seasoned connoisseurs, called “gota de aceite” originating from Spanish meaning “a drop of oil”. According to Ronald Ringsrud[1], this expression have been used by at least three generations of Colombian emerald dealers. And such optical effect may increase the value of a emerald. This phenomena is also called “butterfly wing effect” (efecto aleta de mariposa). So far, such optical effect has only been reportedly found in Colombian emerald from various deposits, including Muzo, La Pita, Coscuez, and Penas Blancas mines.
However, this term is unfamiliar for the public mainly because this phenomena is very rare and evens the stone do show such effect only experienced gemologists and seasoned dealers can tell. Some emeralds with such qualities may be mistakingly identified as oiled or even synthetic owing to the wavy structure and very high clarity.
To better elucidate this phenomena, the “gota de aceite” effect in Colombian emerald is like velvety effect in Kashmir sapphires. Additionally, although such scenes are only reportedly found in Colombian emeralds at present[2–5], it does not mean that emeralds from other localities cannot exhibit “gota de aceite”. As it is well known recently that Madagascar also produces sapphires with velvety effect resembling those from Kashmir.
As material science develops, our understanding of crystal have been advanced gradually especially on different dimensional levels. Those crystals which are visually clean by naked eye and 10X loupe may have several fractures when observed under 100X microscope. Those exhibit no inclusion or fractures may show clear cavities under greater magnification such as 1000X. The so-called perfect crystals still possess certain crystal defect within the crystalline structure, including the point defect, vacancy etc.
Thus, a solid foundation of knowledge of crystalline structure helps a lot to understand the formation of “gota de aceite” effect. Emeralds, belonging to beryl group (Be₃Al₂Si₆O₁₈) and hexagonal crystal system, exhibit hexagonal prism with the cross section perpendicular to c-axis being hexagonal, which agrees the outlines of the “spots” mentioned above. Such external appearances are determined by the internal crystalline arrangement, i.e. the crystalline structure. Fig. 6 shows viewing along c-axis of emerald, and this viewing direction is identical to that of “gota de aceite” effect. Six identical units composed of various element atoms are clearly identified by three crossed auxiliary lines with cyan color. These units are consisted of Si and O as the main framework, consistent with the fact that emerald is a kind of cyclosilicate combined by six Si-O tetrahedrons with one six-fold symmetry axis. Furthermore, as it is known that emerald is colored by Cr and/or V elements, when Cr³⁺ and V³⁺ substitute the Al³⁺ site. Connecting lines between six neighboring Al³⁺ form a hexagonal ring, similar to the outlines of the “spots” as shown in Fig. 7. Hence, “gota de aceite” effect has had a close relation with the crystalline structure of emerald and trace elements coloring the stone.
Filming the scene under microscope is relatively a hard and time-consuming task. Because what you have captured using a camera or CCD is somehow different from what you see directly under microscope. To reproduce the scene to the fullest, we have to capture a suit of pictures at various focal distances in identical position, direction and magnification. Afterwards, all the pictures were stacked using Photoshop into one picture to show every possible details. When it comes to “gota de aceite” effect, filming task becomes more difficult mainly owing to the oily appearance and blurry outlines. As more advanced testing and analyzing instruments are been applied to gemological testing, gemologists have more options and can get more information of the gemstones submitted for testing. However, no matter how advanced or powerful these equipments are, observation under gemological microscope still remains an essential and basic skill and can never be emphasized too much.
3 Conclusion
(1) Owing to the subtle appearance and oily and blurred outlines, meticulous observation is strongly needed to detect “gota de aceite” phenomenon, especially when the stone is very clean and suspected to be synthesis.
(2) Advanced equipments such as FTIR and XRF are very useful to quickly determine the authenticity of samples.
(3) To be emphasized, there is no sound evidence to support that “gota de aceite” effect is unique in Colombian emerald. Gemologists should always be open mind and prepare for the unknown. Hence, solid mineralogical and gemological knowledge are necessary.
(4) Considering the rarity of “gota de aceite” effect, gemologists should work together and keep an eye on the daily testing and collect and share more information on such samples to advance our understanding on this subject.
(5) Last but not the least, nomenclature of “gota de aceite” should be careful and consistent. And it should not be over-appreciated and superlative, in spite of its rarity.
Reference
[1] Ringsrud R. Gota-de-Aceite: Nomenclature for the finest colombian emeralds[J]. Gems & Gemology, 2008, 44(3): 242-245.
[2] Gübelin E J. Gemstone inclusions[J]. Gems & Gemology, 1944-45, 4(12): 174-179.
[3] Gübelin E J, Koivula J I. Photoatlas of inclusions in gemstones[M]. ABC Edition, Zurich, Switzerland, 1986.
[4] Hainschwang T. Extraordinary “gota de aceite” emerald submitted to the Lab. GEMLAB Research Newsletter[EB/OL]. [2008-06]. www.gemlab.net/website/gemlab/fileadmin/user_upload/Research/Gemlab-Newsletter-06-2008.pdf.
[5] Schwarz D, Giuliani G. Emeralds from Asia[M]// G. Giuliani et al., Eds., Emeralds of the World, Lapis International, East Hampton, CT, 2002.
