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Left: The transparent roiled texture in this 4.68 ct. emerald |
Table of Contents More Sample Chapters
A sample chapter from Emeralds, A Passionate Guide.
The Colombian novelist Gabriel García Márquez wrote about love and magic in the classic novel One Hundred Years of Solitude. It is set in the fictional 1840s Colombian town of Macondo, situated in the verdant, semi-tropical jungle of the Andes Mountains, amidst the lushness and decay that is the essence of jungle life. The circus has just arrived in Macondo, as it does every year. The circus leader is Melquíades, a gypsy. Melquíades has brought with him many wonders of nature with which to astound the townspeople, including ice, flying carpets and a telescope. The curious brown-skinned circus leader visits his old friend Aureliano Buendía, and before leaving again, gives him a sextant, a gift that is respectfully accepted. One year passes, and Melquíades returns to Macondo with the circus. Aureliano Buendía seeks him out and reverently returns the sextant to his old friend. He exclaims, “Melquíades, the world is round. Like an orange!”
| [Fig. 1] A 9.20 ct. heart-shaped emerald. In the six upper facets a wavy, softening effect adds texture to the color and clarity expressed by this emerald. |
This scene captures the fascination and wonder that accompanies the discovery of new knowledge. This is exactly the proper use of science: to lead us to amazement of Nature’s creation and to experience a sense of wonder.
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Gota de Aceite
There is no better place to rediscover wonder than in a remarkable phenomenon that occurs inside the finest emeralds. That phenomenon is called gota de aceite (Spanish for “drop of oil,” pronounced “go-tuh day ah-say-tay”). A velvety interruption of the light passing through the emerald, the effect is prized by connoisseurs in the same way they value the velvety texture of a Kashmir sapphire. In both cases, the color of the stone is softened and the internal reflections are spread by numerous microscopic inclusions, reducing extinction and giving a liquid, velvety texture. Gota de aceite is associated with the Colombian emerald, and even in Colombian stones it is visible in only one in a thousand, mainly of fine quality. In six years of informal study of this phenomenon, I have only detected it rarely, personally viewing about 18 good examples, 20 moderate examples and 50 that were muted or indistinct.
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| [Fig. 2] Angular growth structures, photographed at 30x magnification, are typical of the gota de aceite phenomenon. |
The term gota de aceite is also known as the “butterfly wing effect” (efecto aleta de mariposa). Transparent irregularities in the internal crystal seem to be the result of changing and unstable conditions during emerald crystallization. These conditions give rise to both raised hexagonal terminations as well as etched geometric depressions. After their formation, these growth structures are overgrown with more emerald. The growth structure and patterns thus formed (Figure 2) are transparent and diffuse the light within the faceted emerald in a manner reminiscent of a drop of thick oil, hence the name. Since growth structures cause the effect, there is no reason that it could not be found in emeralds from localities outside of Colombia.
This phenomenon was also referred to as calcite precipitation (Figure 5). It was thought to be caused by a temporary lull in the crystallization process of the emerald, allowing small grains of calcite to form that were later overgrown with emerald. The calcite theory is losing out to another explanation of gota de aceite. Close microscopic examination indicates the presence of growth structures instead of calcite, according to researcher John Koivula. He points out that the forms (Figure 3) appear three-dimensional when viewed perpendicular to the plane of their formation, but when the stone is turned sideways, there is no space for them; they look flat. Also, no evidence of calcite was seen, either by microscopic examination or Raman spectroscopy.
| [ Fig. 3 ] The growth structures that cause gota de aceite can be somewhat difficult to see. The photo on the left reveals the angular or hexagonal growth features seen when looking down the c-axis. On the right, the same emerald is shown perpendicular to the c-axis, which reveals the typically narrow bands of columnar structures associated with this phenomenon. |
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In the gota de aceite effect, the etching and regrowth structures form in a plane perpendicular to the c-axis. If the emerald cutter positions this plane roughly parallel to the table of the faceted stone, it will be seen and appreciated by the viewer, increasing the emerald’s value and allure. Sadly, the effect gets wasted when the emerald’s owners and cutters do not recognize the effect and position the zone of gota de aceite to one side or perpendicular to the stone’s table. The difficulty of seeing it in the rough, along with the lack of understanding about it, are other reasons for the great rarity of gota de aceite emeralds in the market.
Researcher and author John Sinkankas noted that beryl crystals can often have many sub-individuals included within. Since these grow along with the original crystal, they are oriented to the c-axis, as in Figure 4.
Confusion of Nomenclature
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[ Fig. 4 ] |
Only among connoisseurs is this phenomenon understood. In Colombia, only experienced dealers recognize the butterfly wing effect; others overlook it. Nowadays, in Europe and the United States, the expression gota de aceite is often used to describe any fine emerald, even if it does not have this effect. Also, the expression “drop of oil” has fallen from use because of the negative connotation of the word “oil” in the last two decades. Yet the term has been used by at least three generations of Colombian emerald dealers.
The first published mention of this effect was in an article by Edward Gübelin, accompanied by a photo, in the Winter 1944–45 issue of Gems & Gemology. He explained it as
a great mass of calcite inclusions (presumably precipitation during growth of the host mineral) which is responsible for the slightly oily appearance of some of the most beautiful and highly priced Colombian emeralds.
Dr. Gübelin’s use of the word “oily” is no doubt a reference to gota de aceite, which can be compared to the “roiled” (like water in cognac) appearance of the interior of a hessonite garnet or a Russian hydrothermal synthetic emerald. Dr. Gübelin recognized how the effect adds beauty and fineness to the emerald. In the 1986 in the book co-authored with John Koivula, Photoatlas of Inclusions in Gemstones, he used the term gota de aceite.
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[ Fig. 5 ] Although Dr. Gübelin did not specifically use the expression gota de aceite, his caption refers to the “oily appearance” of an emerald he documented in the 1940s. This is comparable to the roiled appearance (like water in cognac) of a hessonite garnet. Dr. Gübelin recognized that this effect occurs in some of the finest emeralds. |
The 2.77-carat emerald pictured below is a wonderful example of this phenomenon. It was highly prized and sold to a collector for $8,000 per carat nine years ago. In this face-up view, the softening is especially visible through the table, where there is internal reflection of the pavilion facets.
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[Fig. 6] |
Part of the charm of gota de aceite happens when the stone is moving in your hand, which is impossible to show in a photograph. Moving the emerald reveals how this effect reduces extinction by spreading the areas of internal reflection.
Gota de Aceite: Use and
Misuse of the Term
Confusion of nomenclature with regard to gota de aceite has taken two forms. Because the phrase is typically used only with respect to very fine emeralds, some exceptional stones are labeled with this term even though they do not actually have the effect. The mere fact that the emerald is very fine often inspires the owner or seller of the stone to use gota de aceite as a superlative.
There is also confusion relating to “old mine” emeralds. Old mine is a separate term attributed to rare and fine emeralds, but it refers to the provenance and age of the emerald, specifically those emeralds sent by the Spanish colonies in the New World to Europe and Asia in the sixteenth through eighteenth centuries, along with Swat Valley and Habachtal emeralds of the same era. However, the presence of gota de aceite may wrongly inspire the owner or seller to call the stone “old mine.”
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[ Fig.7 ] Sometimes the gota de aceite effect can only be found by first looking through the pavilion, especially if it is a muted example. In the case of this 3.65 ct. emerald, the effect was visible through the table. Turning the emerald under magnification and varied lighting will reveal its extent. |
Conversations with Colombian emerald dealers and connoisseurs reveal common agreement that the traditional definition of gota de aceite requires the presence of growth structures, either angular or hexagonal, as seen in figures 2 and 3. By definition, the structures causing the effect must be transparent. Because of variances in the size of the hexagonal structures and in the thickness of the zone containing them, the strength of the effect is also quite variable. I recommend that gemologists classify gota de aceite as either “muted,” “moderate,” or “distinct.” To be considered “distinct,” the effect should be clearly visible to the naked eye as the stone is rocked back and forth. Moving the stone is important to revealing the liquid-like softening of the texture that is the hallmark of gota de aceite. In searching for the effect with a loupe or microscope, it is also important to turn the emerald in all directions as in Figure 7.
The Bengali Interpretation
There is a Bengali word, snigdha, that translates as “smooth” or “tender.” Indians have heard that word used in describing certain emeralds, those with the gota de aceite texture. The same word in Sanskrit, according to gem dealer Jaideep Muckerjee, can mean glistening, moist or softhearted: all good descriptors for emeralds with this effect.
A Perfect Storm of Unique Traits
In addition to the butterfly wing effect, Colombian emeralds are blessed with a natural fluorescence that takes in visible light and sends it back to you as a red message of passion underneath the sober and respectable green color. The red is invisible, but it subconsciously grabs your attention: rather like the way a woman who may be dressed in a normal manner is found wearing perfume of the most primal and powerful scent. The man didn’t see anything, but whatever it was, he now finds this person irresistible. The butterfly wing effect is like adding romantic music to the above scene of enchantment and allure. Can anyone resist?
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| [Fig 8 ] A 3.65 carat square emerald cut, magnified 20x, focused 3 mm below the table. This image dramatically reveals how an area of extinction (the dark area at the top) can have color spread across it through the soft dispersion of light coming from the gota de aceite effect. |
Now that we are in the depths of colored stones enchantment, this is a good time to refer back to the writing of passionate ruby expert Richard Hughes. The reason that ruby from Burma (now Myanmar) and emerald from Colombia are at the absolute top of colored stone desirability and price is that they both share properties like the ones mentioned above in remarkably similar manners. Colombian emeralds have not only fine color but natural fluorescence and microscopic inclusions whose presence reduces extinction. The Burmese ruby fluoresces too, and has inclusions, called silk, that spread the areas of color without reducing transparency. Hughes explains the phenomenon:
What gods are these? Not only did they bless the ruby with an inborn glow to match its scarlet skin, but such was their benevolence that they also gave us silk—oriented needles of rutile—gossamer threads that banish the darkness besmirching the rest of the mortal gem world. Such tiny exsolved inclusions scatter light onto facets that would otherwise be extinct (dark). This gives softness, as well as spreading it across a greater part of the gem’s face.
Lest the above discussion give you the impression that the butterfly wing effect only occurs in fine emeralds, the 3.65 ct. emerald in Figure 8 sold for only $1,500 per carat, compared to $8,000 per carat for the 2.77 ct. gem mentioned earlier. This once again reveals the advantage of knowledge in buying. In Bogotá, the dealers were not very interested in the 3.65 ct. emerald because of an off-center keel and a large window. I immediately noticed the butterfly wing effect and purchased the stone at a favorable price. In the white stone paper, the beauty of the stone was less apparent. I knew, however, that in a piece of jewelry the emerald would be magnificent. This slight incongruity in the market allowed for a quick business decision.
Back in the United States, I watched a jeweler as he rolled the emerald back and forth between his fingers, keeping it in constant motion. He was quick to grasp the potential of the emerald for creating a beautiful jewelry piece and bought the stone, thanking me for showing it to him first.
Another lucky discovery was made on a separate trip to Bogotá. A 2.27 ct. emerald was passed over by two buyers because it was windowed. I purchased it with my heart, meaning I didn’t have a specific reason to buy it except that I liked it. It was very clean under the loupe, and the four corners looked sturdy (a very important issue on a flat, spread stone). Only back in the U.S. did I subject it to microscopic examination and discover it had the rare and coveted gota de aceite effect!
Twice in the last year I have discovered emeralds in jewelers’ inventory that contained a distinct butterfly wing effect. I asked both jewelers if the dealer who sold them the emerald had mentioned the effect, or used it in promoting the stone for sale. In both cases, the answer was no—a strong commentary on the need for gem knowledge among professionals and greater appreciation for nuances like the gota de aceite effect.
Other Pleasing Textures
In addition to gota de aceite, there is another set of inclusions that causes a pleasing visual effect. These are fields of semi-transparent inclusions that create a similar scattering of the light. Figure 9 shows the numerous two- and three-phase inclusions that caused an effect reminiscent of gota de aceite. Because the inclusions are not completely transparent, the effect is less distinct but still pleasing. It could be called a pseudo–gota de aceite effect. The true gota de aceite effect requires the angular or hexagonal patterns shown in Figures 2 and 3. They must be transparent as well.
| [ Fig 9 ] A field of semitransparent two- and three-phase inclusions creates a pseudo–gota de aceite effect in this 3.05 ct. emerald. |
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The pseudo–butterfly wing effect is only visible because the cutter oriented the field of two- and three-phase inclusions parallel to the length of the stone. The image shows the effect, but moving the emerald in the fingers would display the velvety, liquid texture that is so pleasing.
Third on the list of textures caused by inclusions are white wispy veils scattering the light within a stone and creating a velvety effect. These veils, which are mostly healed fractures, also spread the color and reduce dark areas of extinction. On the next page is an image of a 6 ct. emerald magnified under darkfield illumination. The wispy veils are highly visible and opaque. The same emerald under regular indoor lighting seems “cleanish” and shows a pleasing velvety texture. Another example of this is found in Figure 3 of Chapter 7.
Inclusions Are Good: Part 2
The three examples have shown us how inclusions are useful in beautifying the visible texture of an emerald. We have also seen, in Chapter 6, how rubies and sapphires have similar sought-after textures that are caused by inclusions.
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[ Fig.10 ] The healed fractures visible in this 6.19 ct. oval-shaped Colombian emerald above are white wispy veils. They are opaque and clearly visible under darkfield illumination. The image below shows how the same white wispy veils are less visible in overhead light. Yet they create a velvety effect that broadens the internal reflections and reduces dark areas of extinction. This pleasing effect is much more common than gota de aceite. |
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Conclusion
The term gota de aceite has now been defined and specified; it is not meant to be a marketing superlative. It is hoped that this chapter will call attention to some inconsistencies of nomenclature in a trade full of arcane terms and meanings. To merit the descriptor gota de aceite, an emerald requires careful examination and an understanding of the growth structures that cause the effect. Other effects cause us to appreciate inclusions in emeralds and in colored stones. If any effect deserved to be put in the category of fancy clarity colored stones, as proposed by gem researcher John Koivula, it would be the gota de aceite effect in Colombian emeralds.
