Green Gold – Columbia’s emeralds
Oh, what a little hot water can do to boring old shale
Before the Spanish conquest of what is now Colombia, people in the mountains north of Bogota are said to have thrown emeralds into Lake Guatavita. Once a year the Indian ruler would cover himself with honey and gold dust and at daybreak have his men row him out into the lake. As he plunged into the water, offering the gold to his god, the crowd on shore would throw in their own offerings. The rich ones chucked in emeralds.
When the Spaniards finally found the Indian emerald mines after decades of bloody searching, the Old World went crazy for the New World’s gems. Although the Egyptians had begun mining emeralds near the Red Sea as early as 1650 B.C.–and emeralds had long been symbols of immortality, cures for dysentery, and preservers of chastity–the new Colombian gems were the clearest, biggest, and greenest anyone in Europe had ever seen. They still are: the same mines remain in operation, accounting for 60 percent of the world’s production.
Emeralds are valuable because they are rare, rarer than diamonds. They are rare, says geologist Alain Cheilletz of the Center for Petrographic and Geochemical Research in Nancy, France, because they are a mixture of elements that don’t ordinarily get a chance to mix: “They are a mineral that shouldn’t exist at all.”
An emerald is a type of beryl, a mineral made of beryllium, aluminum, silicon, and oxygen. All those elements are common in the continental crust, so beryls are not rare. But whereas ordinary beryls are colorless, emeralds are green because a few of the aluminum atoms in their crystal structure have been replaced by atoms of chromium or vanadium. Neither of those elements has any reason to meet up with beryllium; they and it belong to two different chemical families that drifted apart billions of years ago.
Soon after Earth was born, when it was young and mostly molten, a lot of silicon and aluminum rose to the surface, like a kind of scum, then cooled, forming the first continents. Most of the iron stayed behind in the mantle or sank into the planet’s core. Other elements chose one of those two fates, too, based on their weight and size.
Because of this parting of the elements, Earth’s surface rocks are segregated into two realms, like yang and yin: light and dark, crust and mantle, continent and ocean bottom. Geologists call the light minerals felsic and the dark ones mafic. The paradox of the emeralds, as Cheilletz calls it, is that beryllium belongs to the light, felsic, continental side, whereas chromium and vanadium are from the dark, mafic, oceanic side. Emeralds, in other words, are yin and yang in a single crystal. “The whole problem in our research,” says Cheilletz, “was to figure out the geologic conditions that could permit these two elements to meet at the same time and place.”
The answer, they discovered, has to do with plate tectonics, the ceaseless shifting of Earth’s crust that smashes continents together to build mountains. Every now and then, when an ocean disappears between two colliding continents, a chain of volcanic islands or a slab of seafloor gets beached on land. As a result, the continental crust has over the eons lost its original purity; it has become a patchwork that includes oceanic rocks, and thus traces of chromium and vanadium, along with the continental rocks that are laced with beryllium.
To make an emerald, though, those elements have to come together in a single hot liquid. The most common place for it to happen is underneath a young mountain range. Where the edges of two colliding plates stack up, continental rocks can get dunked so deep into Earth that they melt again, liberating a great balloon of magma that rises back through the crust. At a depth of around six miles, the magma reaches its level of neutral buoyancy, stops, and begins to cool and solidify as granite. From the top of this cooling mass, streams of superhot, mineral-laden water–granite juice–migrate upward into fissures in the surrounding rock and begin to leach out elements.
Ninety-five times out of a hundred that surrounding rock is some ordinary bit of continent, and nothing terribly novel happens. “But if by chance the granite happens to hit a zone of mafic rock incorporated in the continental crust, then the chemistry will be completely different,” says Cheilletz. It will include iron, magnesium, and calcium–and traces of chromium and vanadium.” When the felsic-mafic mixture finally freezes, the fissure will be filled with biotite, a kind of mica–black, flaky, and useless. But scattered through the mica, like green snowflakes, may be emeralds.
Most of the world’s known emerald deposits, from the 3-billion-year-old ones in South Africa to the 9-million-year-old ones in Pakistan, were formed by granite intrusions. In the 1980s, Cheilletz and his colleague Gaston Giuliani studied deposits like that in Brazil. Then they went on to Colombia to have a look at the most renowned emerald mines–and soon saw that they didn’t fit the standard picture. “In Colombia, geologists had been looking for granites but not finding them,” Giuliani says. “When I arrived, I saw right away that the rocks were not the same.”
Instead of granites intruding from below, in Colombia there are black shales laid down from above–sedimentary rocks deposited on the floor of a shallow inland sea during the Cretaceous Period, 100 million years ago. The sea must have been shallow, because the shales are sandwiched among layers of salt, which precipitated out of the water at times when it had all but evaporated. Black shales, besides being progenitors of oil fields (of which Colombia has a few), also contain everything that washed off the various rocks that made up the neighboring land. The Colombian shales contain, in dispersed form, all the ingredients of emeralds.
According to Giuliani and Cheilletz, those ingredients came together on two distinct occasions, 65 million and 38 million years ago. Surges in plate motions–the Atlantic Ocean was getting wider, pushing South America against the Pacific and raising the Andes–caused the thick stack of sediments under the shallow sea to buckle. Large sloping faults formed several miles down in the sediments, and hot water was squeezed out of them, escaping upward along the faults. Rising through layers of salt, the 570-degree water became extremely corrosive. Continuing through layers of shale, it dissolved out the emerald ingredients. Finally it pooled under a layer of especially impermeable shale until the pressure became great enough to shatter that layer explosively.
Then the hot solution shot up through empty cracks in the rock. As its temperature and pressure plummeted, emerald crystals snowed out of it immediately. It all happened so fast, says Giuliani, that the emeralds had no time to grow around grains in the surrounding shale. They grew unconstrained and pure, without the minerals that often cloud emeralds found in other parts of the world. That is why Europeans were so enraptured with the Colombian stones when they first laid eyes on them in the sixteenth century.
Like other emeralds, those from Colombia contain tiny pockets of fluid, typically no more than a hundredth of an inch across–gardens, as they’re called in the gem trade. If you look at one of the Colombian gardens under a microscope, says Giuliani, you will see that it contains a crystal of salt, ordinary sodium chloride. The crystal is a trapped fossil of the brine from which the emerald itself crystallized, tens of millions of years ago.
Except for those inclusions, emerald manufacturers today are able to mimic natural processes so well that it can be difficult for a layman to tell synthetics from the real thing. Perhaps that’s one reason emeralds don’t pack the same emotive resonance for us that they did for bygone Indians and kings. We no longer see links to divinity or immortality in an emerald’s limpid green depths. What we might imagine swirling around in the stones is history: the entire history of the planet distilled into a single miraculous (scientifically speaking) crystal. That’s resonance enough for a rock.
COPYRIGHT 1999 Discover
COPYRIGHT 2000 Gale Group