By Mark Leatherman
Traditionally, the color purple is associated with royalty that has stemmed from ancient Phoenicia (now the nation of Lebanon). It is there, in the Tyre region of the Mediterranean Sea, that a very rare oyster makes its humble abode and produces the purple dye. It would take more than 9,000 of these clams to produce just one gram of the colorant.
The rarity commanded high prices that only the most elite could afford. With all due respect to amethyst, perhaps the color that most symbolizes royalty in the mineral and gem realm is green; more specifically, the green that is tied to the mineral group beryl that gives us emeralds. To commemorate May’s traditional birthstone, we shall explore what naturally makes emerald worthy of royal status.
Emerald is the greenish variety of the beryl group (Be3Al2Si6O18), a cyclosilicate that also includes the gems aquamarine, goshenite, heliodor, red beryl, and maxixe. I include the -ish ending since emeralds can be a variety of hues, also because there are green beryl that are not “true” emeralds (more on this later). A more technical color definition for emeralds is the beryl that bear green, bluish-green, or yellowish-green hues. Due to its chemical composition, beryl is frequently associated with pegmatites. Although pegmatites are not the most common igneous rocks, they are still far from rare enough to justify the gem’s scarcity. To answer this, we must look at both the majority and trace element chemistry in a small game of “which of these things does not belong?”
The two primary chromophores of emerald are chromium and vanadium, with iron playing a role in some cases. In the February 2020 edition of Rock & Gem, I went into detail about why certain sets of elements occur with each other, with their proximities on the Periodic Table being the abridged version. Pegmatites, being felsic rocks (tied to “light” colors) enriched in silicon, aluminum, sodium, and potassium can also source beryllium thanks to its proximity to the latter two elements. The two coloring agents are transition metals in the middle of the Periodic Table, away from the “felsic ends,” being very close neighbors to iron; a key element in mafic (dark-colored) igneous rocks. Thus, emeralds need ingredients from both felsic and mafic rocks, preferably close in proximity, quite a rare condition in the field.
Additionally, even with rocks that are “yin-yang” in nature, an additional obstacle is the inherent rarity of beryllium, chromium, and vanadium in the earth’s crust. The average crustal abundance of the three elements is around 3, 92, and 97 ppm, respectively. Lastly, one must consider a narrow range of precipitation conditions (i.e. crystallization temperature, pH, etc.). The contrasting and scarce ingredients and strict environmental conditions are the prime drivers in generating emerald’s “natural royalty.”
Understanding Emerald Pigmentation
In zeroing in on the causes of pigmentation, the quantities and proportions of chromium, vanadium, and iron dictate the specific hue, and thus richness, emeralds take on. In looking at the “red-headed stepchild” of the beryl family, the moniker “green beryl” alludes to a pale appearance wherein iron (both in Fe+2 and Fe+3 forms) is, by far and away, the dominant colorant.
Given that iron is far more abundant in the crust than Be, V, and Cr, green beryl is far easier to find than even non-gem quality emeralds. The world-famous Colombian emeralds owe their specific tinge to a total lack of iron while being chromium-dominant, whereas Zambian emeralds (arguably Colombia’s chief rival on emerald quality) is also chromium dominant but display a blue undertone due to trace amounts of iron. In adding to the bulk requirements for nature to synthesize the gems, the proportions of multiple chromophores have to be like Goldilocks; just right!
With the richness of color being the primary driver of these gems on the world market, the second-place value factor is the degree of inclusions emeralds contain. If one is new to the world of the green gem, one of the most important principles one should learn right away is that inclusions are acceptable (unlike how diamonds in the jewelry industry are treated).
Emeralds are classified as a type III gemstone, meaning inclusions are very common and acceptable (type I gems are inclusion-free more often than not, such as topaz and aquamarine, and type II gems are usually included such as rubies and sapphires). In fact, a unique set of inclusions an emerald gem has is often referred to as its “jardin” (Spanish for “garden”, as the term is most often used in Colombian emeralds). Such inclusions are typically elongate and rod-like, resembling tree branches and roots, hence the moniker. So, if an “inclusion-free” emerald is spotted on the market, you are probably dealing with a fake. One additional question that some may have is, “how can aquamarine and emerald be on opposite ends of the inclusion spectrum if they are both beryl?”
The answer lies in the environments both gems form in 1) emeralds are found in sedimentary shale-limestone sequences and boundary zones between pegmatites and high-grade metamorphic rocks such as schists. And 2) aquamarines are generally confined to just pegmatites. The shale-limestone sequences usually involve a brine solution where a variety of dissolved elements and inclusions persist, and the boundary zones involve the exchange of multiple ingredients from each source (so it gets quite crowded).
Individual pegmatite melts are often supercritical fluids, meaning that distinct liquid and vapor phases no longer exist. These fluids are generated at more than around 375°C and 240 atmospheres of pressure, for water, and possess enhanced solubility of a pure liquid yet the permeability of pure gas. Due to especially the latter, when some supercritical pegmatites cool to form gems (like type I aquamarine), there is high selectivity of what gets included in the developing crystal structure (hence giving a higher purity).
Emerald World Tour
With the background now cleared up, the “emerald world tour” can commence. As a disclaimer, not every single world emerald locale can be mentioned, and what better place to start than the ultimate classic location of Colombia. So far, the Colombian deposits are the only well-known examples of the gems forming in the shale-limestone sedimentary environment. The famed locality is divided into western and eastern belts separated by 110 kilometers, where the acclaimed Muzo and Chivor mines reside, respectively. The gems have been worked there by indigenous tribes in the 4th century, with Conquistadors arriving in the early 16th century undertaking more extensive mining. It did not take long for them to realize that their finds would be more attractive than Egyptian emeralds back in Europe (their closest source).
The regional geology is composed of lower Cretaceous-age organic-rich shale and limestone beds that have been intensely folded and intruded by reverse faults during an orogenic event that built up the Colombian Andes. With it being such a unique and important locality to the worldwide emerald market, there have been numerous proposed models to explain the gem formation. In general, the consensual miracle geologic recipe boils down to multi-step fluid reactions, rich carbon contents, presence of evaporite sedimentary rocks, and space to grow.
Seated deeper than the noteworthy shale and limestone formations are sequences of evaporitic sedimentary rocks rock salt (NaCl) and rock gypsum (CaSO4*2H2O) — the term evaporitic comes from the formation of these rocks, via high evaporation rates of briny waters, that leaves behind the minerals when the remaining solution becomes supersaturated with the dissolved elements. Such past environments are typically deserts and lagoons.
During mountain building, deep-seated hydrothermal fluids are forced upwards along faults, while interacting with the evaporitic rocks along the way. These fluids also contain Be-F complexes derived from basement granitic rocks. As they arrive closer to the surface, where the shale-limestone sequences are, the interaction with the carbon in the organic-rich shale reduces the dissolved sulfate (SO4-2) and iron to produce pyrite (FeS2) and elemental sulfur. The transported fluorine also reacts with the calcium in the shale-limestone to produce fluorite (CaF2).
Additionally, the removal of aqueous iron into pyrite is key. Iron is a common minor chromophore in other worldwide emerald locales. The signature grass green hue in Colombian emeralds is from the fact they are virtually iron-free. Without a strong reducing agent, like the organic-rich shale, the iron would partially substitute for beryllium, giving a bluer shade that predominates Zambian emeralds.
Before moving on to the aforementioned African emerald leader, Colombia offers up two more ultra-rare surprises. Due to the abundant organic matter that dominates the shales, it goes without saying that it is a common inclusion. However, if the said matter gets entrapped in the growing crystal at the right time, flakes can orient themselves along crystallographic axes intersections. If this is combined with an inconsistent growth rate of the gem, a unique pattern called trapiche emeralds form (the word also refers to a Colombian grinding wheel), where the trapped carbonaceous matter divides a complete crystal into six geometrical areas roughly resembling trapezoids with crystallizing albite often filling in the boundary areas.
The other Colombian surprise is rarer even yet, in the form of fossil replacements. If fossils are lucky enough to be rapidly buried after their demise, their shells can be replaced by minerals such as hematite, pyrite, quartz, and calcite. A lucky relatively few gastropod fossils found in the shale beds here have had their shells replaced by emerald.
Perhaps Colombia’s main contender for gem-quality emeralds, especially in jewelry, lies in the African nation of Zambia. The Zambian deposits occur in a rather rare host of talc-magnetite-actinolite-chlorite schist that was born out of a metamorphosed komatiite (ultramafic lava flows that are too hot to erupt today) around 1.6 billion years ago. The komatiite provided the chromium, along with iron. Fast forward to 530 million years ago during the Pan-African orogeny, which saw the development of pegmatite igneous intrusions, that provided the beryllium. The emeralds then grew in a “meeting” or reaction zone along the boundary between the two rock types. Thus, many Zambian gem specimens show a quartz matrix, with some showing solely a mica matrix, but specimens showing both on the matrix are exceptionally rare.
Differential Details Between Emerald Types
A rather important market point of Zambian versus Columbian emeralds is that the latter is
exceptionally more valuable despite those gems being more included. This goes to show that a desirable color, and the historical “brand name” of Colombian present, takes precedence over having much fewer inclusions but with a slightly “less ideal” green that Zambian gems bear. Of course, the term “less ideal” is rather subjective as the blue-green African variant has been gaining popularity in the rockhounding realm. Thus, if you are looking to add a first gem-quality emerald specimen to your collection, a Zambian specimen is a rather affordable first choice.
Another fond example of somewhat-odd emerald schists is translucent to opaque gems housed in biotite schists accompanied by a minor, but showy molybdenite (MoS2). Although I have heard of Brazilian emeralds before, it wasn’t until the fall Denver shows in 2018, that I first enjoyed an up-close glimpse of this very cool and rare assemblage, being sold by my good friends, William and Mandi Hutchinson, of Alta Gema. More specifically, the origin of these specimens is from the Carnaíba Mine.
The Carnaíba is a part of the largest emerald deposit in the world, in terms of physical size, and was only been discovered in the 1960s. Brazil is the source for roughly 10% of world emerald production, with other locales such as Minas Gerais, contributing alongside Carnaíba. Emeralds here tend to be heavily included, and rarely transparent, but still make desirable matrix specimens.
As said above, the matrix is Precambrian mafic biotite schist that is in proximity to pegmatite intrusions of Proterozoic age. The schist formations themselves are spatially close to chromite mines further to the west, where the chromium chromophore got remobilized during matrix rock intrusion, with the beryllium being delivered from the pegmatite.
Although Carnaiba emeralds are far from Colombian gems, in terms of overall desired gem color and clarity, they are still nothing to shake a stick at as some major record-breakers have been mined from this area. The first was discovered in the summer of 1974 when a cut specimen weighed in at a whopping 86,136 carats (nearly 38 pounds)!
As a side note, the largest natural uncut single emerald crystal was found in Colombia (1969) weighing in at 7,205 carats (nearly 3 pounds), called the Emilia Crystal. In more recent times, Carnaíba struck the record books again when a 794-pound, 4.3-foot high, matrix specimen was unearthed in 2017. It took a team of ten, a week, to transport the specimen from 200 meters (656 feet) underground. With an estimated value of $309 million.
Soon after the extraction, the exact location and owner of the massive specimen were understandably made secret due to fears of it being burglarized. The specimen serves as a very close cousin to the infamous Bahia emerald, which was found in 2001 and only 100 meters (328 feet) away. The Bahia was once a subject of a decade-plus long legal battle over its rightful ownership, but there is not enough space here to get into all the tale’s intricacies. If gem transparency is not of the highest importance, but your wallet certainly is, you cannot go wrong by adding a Brazilian specimen to your collection.
Somewhat like Brazilian emeralds are those produced from the Dayakou Mine in southern China. As with Carnaíba, the Dayakou emeralds are hosted in “traditional” mica schist and are typically opaque and heavily-included. The geological scene was initially set by Proterozoic metamorphic basement, followed by the emplacement of Silurian-age gneisses and granites, and finally by the intrusion of Cretaceous-age pegmatites supplying heat and beryllium for emerald formation.
The Chinese emeralds here are cool for three reasons: 1) vanadium, rather than chromium, serves as the dominant chromophore. 2): the gems precipitated out in two different vein orientations; one trending northwest that formed during the Silurian, and the other trending northeast formed during the Cretaceous. Thus, it is plausible that there were two separate gem-forming episodes. And 3) a unique association with scheelite (CaWO4), giving some areas of some specimens a nice lemon-lime contrast.
Trading Information for Emeralds
My first encounter, and subsequent purchase of a Dayakou specimen, was in June 2012 at a local show in Bloomington, Indiana, when I was a graduate student. During some downtime in running a student-led geology club fundraising table, I perused the other dealers’ offerings when I came across one with a plethora of Chinese matrix specimens. I looked at them in awe, wishing I could afford one (poor graduate student problems). After introducing myself, the dealer (Dan) asked for my input on the geochemistry of some Russian carrolite crystals he was also selling. He had some basic background knowledge on their formation, but in learning that I specialized in geochemistry, he asked if I had any deeper insight.
After regaling him with information on probable source rocks and trace elements, he said: “You were really eyeing those emeralds, right?” After nodding, he made me a sweet deal on one of his better examples. When he offered me half-off any specimen I wanted, as a thank you, I was elated in that I could afford a sweet five-inch long matrix specimen complimented with some pyrite. To this day, it still resides in my case. A closing observation I have made is that I have not seen much in the way of Chinese emerald specimens since then. Perhaps it is because I’ve been limited to visiting shows in the Midwest and Colorado, but I could certainly be wrong. With the nation’s economy experiencing recent growth, maybe more specimens will be brought to market if further overall mining activity is encouraged.
In considering regions a bit further northwest, there are the famous Mingora deposits in northern Pakistan. The Mingora Mine is a part of the nation’s most prolific emerald producer, being the Swat Valley District. Although the emeralds here are derived from a classic convergent plate boundary setting, Mingora emeralds are unique from a big-picture perspective. They are confined to an ophiolite mélange zone (otherwise known as a suture zone).
Ophiolites are slivers of “foreign” lithosphere (crust) that get compressed or stitched, onto the edges of continents, at convergent-ocean plate boundaries where subduction occurs. The “gluing” occurs since the sliver is too thick to be subducted and is often composed of a myriad of rock and sediment types in a chaotic assemblage (the word mélange is derived from the French word to mix – “mesler”). The mélange zone is known as the Indus Suture situated between the Eurasian plate and the Kohistan Island Arc. This specific mélange contains magnesite (MgCO3), chromium-rich and dravite tourmaline, fuchsite – K(Al, Cr)2(AlSi3O10)(OH)2, serpentinite, and talc-dolomite schist.
In parts of the oceanic crust, chromium is typically found in elevated amounts at its base where gabbroic rocks predominate. Hydrothermal activity and element mobilization are generated by the creation of new magma from the subduction process, with gem deposition found sporadically and within numerous thrust faults created by compressional forces in the mélange.
Emeralds in America
In ending our journey in North America, the longstanding epicenter of United States emeralds is in North Carolina. Colombia and Zambia may have the best gemmy emeralds, and Brazil the largest, but those from the Tarheel State may have the most mystique to them. The deposits are split between two areas: 1) around the town of Hiddenite, and 2) the Crabtree Mine, outside of Little Switzerland 90 miles to the west, and the occurrences are vastly different.
The Hiddenite area has been responsible for the biggest and best finds (so far) in North America. The area can be further divided into the Emerald Hollow Mine, the North American Emerald Mine (the NAEM, a.k.a. the Rist Mine), and the Adams Mine. The general setting places gems in quartz veins alongside siderite, muscovite, calcite, dolomite, albite, and rutile all contained within intensely folded biotite gneiss and schistose rocks. A good number of emeralds (not all) I’ve seen from this locale bear a distinctive light mint hue. Also found in the area is the titular gem spodumene that also gets its color from trace chromium. The first piece of mystique here is that hiddenite and emerald are rarely found together in the same localized cavity or vein, despite needing the same colorant.
Also, the entire area seems like a mystery in that the source of the beryllium and
chromium is still unknown. In any case, the lack of scientific answers has not quenched the desire for discovery on all levels. Since operating the NAEM in 1998, the regional king of emeralds, Jamie Hill, has extracted more than 20,000 carats of crystals. The very next year, more than 300 of those carats were cut into legendary beauties such as the 18.8 ct. “Carolina Queen” and the 7.85 ct. “Carolina Prince.” The former was appraised at a million dollars, while the latter set the record as the most paid per carat for a North American cut gem (just shy of $63,700/ct.). The green luck struck again four years later when a record 1,869-carat crystal was successfully extracted and sold to the Houston Museum of Natural Science.
Lastly, in 2006, Hill made history again with the longest crystal extracted, measuring ten inches. Subsequently, it sold for $155,000 during a Beverly Hills auction. One more record was set, in August 2009, at the Adams Mine, where Terry Ledford unearthed a 310-carat specimen that would yield the continent’s largest cut emerald (at nearly 65 carats) called the “Carolina Emperor.” Despite the recent success stories, a systematic way of exploring for the region’s gems is lacking, with veins and pockets randomly coming and going, adding again to the mystique (and drive).
The only mine in the region where rockhounds can try their luck at finding the green gem is at the Emerald Hollow Mine (www.emeraldhollowmine.com). In general, North Carolina emeralds are typically not known for being as large as their international cousins. Thus, when the next record-keeper is unearthed, it is guaranteed to generate quite the headliner in the rockhounding community.
The Crabtree Mine area possesses an entirely different stage for finding gems and the form they come in. Like with most of the discussed locales, the gems grow along a boundary zone between pegmatite and biotite schist. Finding solid three-dimensional crystals is quite tough, whereas most of the eye-popping specimens are displayed as small patches of green en cabochon, alongside feldspar, quartz, black tourmaline, and garnet. The claim is managed by the Mountain Area Gem and Mineral Association (M.A.G.M.A.), and collecting visits can be arranged by visiting wncrocks.com.
I had the good fortune of visiting the grounds during my first honeymoon back in 2010. We were warmly received by the M.A.G.M.A. members, and the tailings were plentiful. There was no need for us to dig any massive holes. We did not have quite the luck we were hoping for, but we walked away with a few small pieces containing small bits of green (the largest being an inch in width). One of the specimens was a skinny ten-millimeter yellow beryl crystal nicely tucked within layers of biotite flakes. It also probably did not help that mosquitos were out in full force on a hot July day. I look forward to the day that I may return once more.
With probably the most erratic and random geological distribution, save for probably diamondiferous kimberlite pipes, the precious green gem has captivated humanity for millennia, for better and for worse. It’s only a question of where the next major emerald deposit will be located; it will be striking the geological lottery. One thing is for sure though; this author is lucky enough to have emerald as his birthstone.