Emerald Analysis a Precious Tool for History
Many of the most precious stones in history’s jewelry box are emeralds.
Indeed, when the Roman emperor Nero sought to shield his eyes from the noonday glare, he donned sunglasses with emerald lenses. The moguls of India inscribed the green stones with sacred texts. Ancient Egyptians tucked them in the wraps of royal mummies as talismans of immortality.
Today, most emerald buyers may be more interested in a gem’s price than the mine from which it was quarried or the trade routes it followed to market.
But for curators tracing the emeralds in the Holy Crown of France or recovered from a sunken galleon, such questions are of pressing historical interest. Mineralogists are eager to uncover the geological conditions under which heat and pressure distilled each gem’s distinctive mix of beryl, vanadium and chromium into hexagonal crystals.
“The problem is one of provenance,” said Caltech mineralogist George R. Rossman. “Where do archeological materials or gemstones come from? It is a difficult problem to solve because much of the original information has been obfuscated or lost as time has taken its toll.”
Until now, that is.
Researchers in France and Colombia for the first time have found a unique geochemical signature in historically important emeralds that unerringly reveals their origins.
Their results are helping researchers reconstruct ancient trade routes, while providing hints of long-lost emerald mines.
Published recently in Science, the study revealed the surprising extent of Colombian emerald trade in Europe and Asia soon after the seizure of New World mines by the Spanish. It also discovered previously unknown Old World sources of emeralds.
The technique, called ion microprobe analysis, has now been used to measure oxygen isotopes in nine of the world’s best-known emeralds. The stones span a time from the Roman occupation of France until the 18th century.
By calculating the ratio of one oxygen isotope to another, the researchers identified telltale variations in the chemistry and temperatures in the superheated fluids from which the emeralds crystallized.
The result, said lead researcher Gaston Giuliani at the Center for Research in Petrography and Geochemistry in Nancy, France, is an “identity card for emerald” that accurately pinpoints the mineral deposits from which an emerald was mined or collected.
“A lot of the scientific mystery of these stones will start to fall away,” said Jeffrey Post, curator of the National Gem and Mineral Collection at the Smithsonian Institution, which contains more than 300,000 gem, mineral and crystal specimens.
“It adds a nice tool to the analytic arsenal,” Post said, because the new technique “tells us something about these materials as gemstones but also as minerals and crystals.”
The technique is confirming some myths and debunking others.
Although Egypt and Austria were thought to be the only sources of gem-quality emeralds in the ancient world, the researchers determined that an emerald from an ancient Roman earring at the French Natural History Museum actually originated in Pakistan, revealing a previously unrecognized ancient source.
In the same way, the researchers showed that a large emerald from the princely coffers of the Nizam of Hyderabad in India--thought to date to mines worked in the time of Alexander the Great--originated instead in Afghanistan.
The emerald was one of four that had been cut by Indian jewelers in the 18th century. Tests showed that the other three so-called “old mine” stones in the Nizam’s treasury had oxygen ratios indicating they came from Colombia.
In their study, the French and Colombian researchers also traced an emerald from the Holy Crown of France to a mine in Austria. The stone apparently was unearthed there about 500 years before the first recorded discovery of those deposits. Two gemstones in the French Natural History Museum, used in 1806 as the benchmark for the mineral content of all emeralds, were traced to mines in Egypt.
One rough-cut emerald recovered from the wreck of the Spanish treasure galleon Nuestra Senora de Atocha, which sank off Florida during a hurricane in 1622, came from Colombia, underscoring how rapidly conquistadors exploited the emerald mines of the New World.
In each case, the emerald was bombarded with an electron ion beam that dislodged oxygen ions from the crystal lattice of the gem, which could then be analyzed.
The test causes only microscopic damage to the emeralds, Giuliani said, so he had little difficulty convincing curators that their priceless emeralds would not suffer any real harm.
Even so, James Shigley, research director of the Gemological Institute of America in Carlsbad, said he doubted that the technique would ever come into general use because it does require some material--however microscopic--to be removed from the jewel.
“It is a kind of destructive testing,” Shigley said. “Traditionally, gemological testing has shied away from destroying people’s gemstones in some way.
“It has great scientific value, but it is not so practical for the emeralds you encounter in the jewelry store,” Shigley said.
University of Georgia geologist Norman Herz developed a similar technique several decades ago to trace the geochemical origin of the marble quarried for classical statuary. Only now have scientists started to apply the idea to jewels, said USC mineralogist Jean Morrison.
“This is a very practical application using detailed mineralogical and chemical composition to infer how emerald trade routes went,” she said. “I think it is kind of cool.”
Rossman at Caltech and geologist John Valley at the University of Wisconsin in Madison have been working independently to apply the same analytical techniques to chemically fingerprint sapphires and rubies.
“I am looking for clues to the geological processes,” said Valley.”If we can show that gem-quality sapphires only form in a certain environment [with] a distinct isotopic signature, this would be an important prospecting tool. It would tell you where to look for new gem resources.”
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History’s Jewelry Box
By analyzing isotopes of emeralds, researchers have been able to determine where they were mined. A new study of famous emeralds revealed the surprising extent of the emerald trade from Spanish mines in the New World, and uncovered sources of ancient emeralds.
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Scientists have learned that the legendary Silk Route was used to trade emeralds from Egypt, Pakistan and Habachtal between Asia and Europe, and that beginning in the 16th century, the Spanish traded emeralds accross the Atlantic Ocean and then across the Pacific.
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National Treasures
These emeralds are among more than 300,000 gem, mineral and crystal specimens in the Smithsonian National Museum of Natural History’s collection.
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Source: Smithsonian Institution
Photos by Chip Clark, Smithsonian Institution
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