Emerald

• 1. Introduction

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  Emerald is a green gemstone, and a single trace metal decides whether it exists at all. Strip the chromium, or sometimes the vanadium, out of the mineral beryl and the green simply vanishes. That fragile chemistry sits inside a stone the gem trade calls one of the traditional big four precious gems, ranked beside diamonds, rubies and sapphires. Yet an emerald is a difficult companion. It cracks easily, it hides a tangle of internal flaws, and most of the emeralds sold today have been quietly soaked in oil before reaching a buyer. How does a stone this fractured become this coveted? Why is its green graded by the naked eye when a diamond demands a loupe? Who first pulled emeralds from the ground, and who learned to grow them in a laboratory instead? And how did a brittle green crystal end up engraved with scripture, blamed for a king's death, and built into a fictional city ruled by a wizard?

• 2. A Flawed Green Crystal

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  Beryl, the parent mineral, carries the formula Be3Al2(SiO3)6, and emerald is its green variety. Chromium or vanadium supplies the color; iron joins them as a third trace element, and the presence, absence and balance of all three set the exact shade of any given crystal. As a cyclosilicate, emerald tends to form in the company of quartz, muscovite, albite, schorl, microcline, fluorite, smoky quartz and elbaite.

  A hardness of 7.5 to 8 on the Mohs scale sounds reassuring, but hardness is not the whole story. Most emeralds carry many inclusions, which leaves their toughness, meaning their resistance to actually breaking, classified as generally poor. The stone resists scratching while remaining prone to fracture.

  The inclusions and surface-reaching fissures laced through an emerald even have a name borrowed from French. Gemologists call them jardin, meaning garden, because the mossy webbing looks like foliage trapped inside the crystal. Those imperfections are unique to every stone, which means a particular emerald can be identified by its own private pattern of flaws.

• 3. The Naked-Eye Standard

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  Color, in gemology, splits into three separate components: hue, saturation and tone. Emeralds run in hues from yellow-green to blue-green, but the primary hue must read as dark green, with yellow and blue allowed only as the normal secondary hues. A grayish cast is the usual saturation mask, and gray turns a green dull.

  Tone draws a hard boundary around the very word emerald. Only stones medium to dark in tone qualify; a light-toned crystal is demoted to the species name green beryl instead. The finest emeralds sit at roughly 75 percent tone, on a scale where zero percent is colorless and 100 percent is opaque black.

  Clarity is where emerald breaks the rules of its rivals. Diamonds are judged at 10 times magnification under a loupe, but emeralds are graded by eye. An emerald with no inclusions visible to a person of normal eyesight counts as flawless, even if magnification would reveal more. Stones with no surface-breaking fissures at all are extremely rare.

  That rough, non-uniform interior shapes how the stone is cut. The signature emerald cut is a rectangular shape with facets running around its top edge, and the oval cut is also common. When a crystal is too included for facets, cutters turn instead to the smooth domed cabochon. The highest prices go to eye-clean stones of vivid green, carrying no more than 15 percent of any secondary hue.

• 4. Soaked In Oil

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  Because nearly flawless emeralds barely exist, almost every one is treated before sale, and the standard treatment is oil. Cutters fill the surface-reaching cracks with cedar oil, chosen because its refractive index closely matches the stone, so the fractures seem to disappear and the gem becomes more stable. Synthetic oils and polymers such as Opticon serve the same purpose, and the cheapest, most fractured emeralds are filled with epoxy resins.

  The filling is coaxed in under pressure. Stones are placed in a vacuum chamber under mild heat, which opens their pores and lets the filler soak deeper into the cracks. The practice is old and largely accepted by the trade, but it carries a price: an oil-treated emerald is worth far less than an untreated stone of similar quality.

  Regulators draw a line at honesty rather than at the oil itself. The U.S. Federal Trade Commission requires sellers to disclose that an emerald has been oil-treated, and untreated stones must travel with a certificate from a licensed, independent gemology laboratory. Green-tinted oil, which adds color rather than clarity, is not acceptable in the trade.

  Enhancement gets its own four-step scale: none, minor, moderate and highly enhanced. Those grades measure how much a stone was improved, not how clean it is, so a gem graded none can still show visible inclusions. Laboratories apply the criteria differently. Some treat any trace of oil or polymer as enhancement, while others ignore oil that does not improve the look of the stone.

• 5. Mines And Origins

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  Mining for emeralds reaches back to Ancient Egypt, where stones were pulled from Mount Smaragdus from 1500 BC, while India and Austria yielded them from at least the 14th century AD. The Roman and Byzantine Empires worked the Egyptian mines on an industrial scale, and Islamic conquerors followed. Egyptian mining only ceased once the Colombian deposits were found, and today only ruins remain in Egypt.

  Colombia stands as the historic largest producer in the world. For the decade leading up to 2005 it accounted for 47 percent of global output, a share that has since declined through a lack of new discoveries, outdated mining technology and global oversupply. Its three main mining areas are Muzo, Coscuez and Chivor, and Colombia is the source of the rare trapiche emerald, marked by ray-like spokes of dark impurities.

  Zambia ranks as the second biggest producer. Its Kafubu River deposits, the Kagem Mines about 45 kilometers southwest of Kitwe, supplied 20 percent of the world's gem-quality stones in 2004. In 2020, Kagem Mining Limited produced 1,880 kilograms, or 9.4 million carats, of emerald and beryl. Zambian stones run bluish green to green, with superior transparency and fewer inclusions than the heavily fractured Colombian crystals, which are prized instead for their intense hue.

  The stone turns up across the globe, in countries from Afghanistan and Australia to Russia, Tanzania and Zimbabwe. Within the United States, emeralds have been found in Connecticut, Montana, Nevada, North Carolina and South Carolina. In 1998 they were discovered in the Yukon Territory of Canada.

  The question of where a loose emerald came from has become its own science. American gemologist David Cronin and his colleagues used energy-dispersive X-ray spectroscopy to read the chemical signatures left by fluid dynamics and precipitation, showing that stones from one mine are chemically alike while stones from different mines differ statistically, even across Muzo, Coscuez and Chivor.

• 6. Grown In A Lab

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  Carroll Chatham ran the first commercially successful emerald synthesis, likely using a lithium vanadate flux. His stones betray their origin by what they lack and what they carry: no water at all, plus traces of vanadate, molybdenum and vanadium. The result is chemically and gemologically the same as a natural emerald, which is why such stones are called created rather than fake.

  Pierre Gilson Sr. became the other large producer of flux emeralds, his products on the market since 1964. Gilson grew his crystals on natural colorless beryl seeds coated on both sides, and growth crept along at 1 millimeter per month. A typical seven-month run yielded emerald crystals 7 millimeters thick.

  The hydrothermal route took a different path. The first satisfactory commercial product came from Johann Lechleitner of Innsbruck, Austria, reaching the market in the 1960s under the names Emerita and Esmeralda, grown as a thin emerald layer over natural colorless beryl. From 1965 to 1970 the Linde Division of Union Carbide made fully synthetic emeralds by hydrothermal synthesis. Their patents, credited to E.M. Flanigen, insisted on acidic conditions to stop the chromium colorant from precipitating out.

  Tairus is the largest hydrothermal producer today, and it has matched the chemistry of emeralds from Colombia's alkaline deposits, so its stones go by names like Colombian created emeralds. Telling natural from synthetic is not simple. Ultraviolet light offers only a supplementary test, since many natural emeralds are inert to it and so are many synthetics. The Federal Trade Commission keeps strict rules here too, allowing a word like synthetic only when the product shares essentially the same optical, physical and chemical properties as the natural stone it names.

• 7. Poison, Scripture, And Oz

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  Emerald is the traditional birthstone for May and the gemstone tied to the astrological sign of Taurus. Around it gathered a thicket of alchemical belief. Old lore held that the emerald counteracts poison, that a venomous animal looking at one would go blind, and that the stone guarded against epilepsy, cured leprosy, strengthened sight and memory, and would shatter if worn during copulation.

  Hernan Cortes carried the lore into history with a stone looted from Mexico. According to the French writer Brantome, who lived from about 1540 to 1614, Cortes had one of these emeralds engraved with the Latin Inter Natos Mulierum non surrexit major, rendered as Among those born of woman there hath not arisen a greater, a line from Matthew 11:11 referring to John the Baptist. Brantome judged it sacrilege to carve so simple a product of nature, and blamed the act for Cortes losing an extremely precious pearl in 1541 and even for the 1574 death of King Charles IX of France.

  The color seeped into fiction and faith alike. In L. Frank Baum's 1900 children's novel The Wonderful Wizard of Oz, and the 1939 MGM film, the protagonist must journey to an Emerald City to meet the Wizard. At the Meenakshi Amman Temple in Madurai, one of India's most famous temples, the idol of the chief goddess Meenakshi is traditionally believed to be made of emerald.

• 8. The Great Stones

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  Zambia produced one of the largest named stones, the Chipembele, a 7,525-carat crystal weighing 1.505 kilograms, recovered in 2021 and held by the Israel Diamond Exchange, Eshed Gemstar. But raw scale belongs to Brazil. The Bahia Emerald, found in 2001, holds 180,000 carats of crystals in a host rock weighing 752 pounds, or 341 kilograms, and ended up in the hands of the Los Angeles County Sheriff's Department.

  Colombia supplied a roll call of historic giants. The Duke of Devonshire Emerald, 1,383.93 carats uncut and dated before 1831, sits in the Natural History Museum in London. The Gachala Emerald, 858 carats uncut and found in 1967, rests in Washington's National Museum of Natural History, which also holds the Chalk Emerald, cut to 38.40 carats and then recut to 37.82. The Mim Emerald, a 1,390-carat uncut, 12-sided crystal found in 2014, lives in the Mim Museum in Beirut.

  Some stones carry deep dates and far-flung homes. The Mogul Mughal Emerald, cut to 217.80 carats, is inscribed with the year 1107 A.H., meaning 1695 to 1696 AD, and is kept at the Museum of Islamic Art in Doha, Qatar. The United States contributed the Carolina Emperor, found in 2009 at 310 carats uncut and cut down to 64.8 carats, now in the North Carolina Museum of Natural Sciences in Raleigh, where the green that began as a few stray atoms of chromium ends as a museum centerpiece.