Origin And Trade Of Some Of The World's Most Historic Emeralds

For more than 4000 years, emeralds have been treasured around the world as a symbol of eternal spring and immortality, prized by the Egyptians, Romans (Emperor Nero surveyed the gladiators through an emerald monocle), the Moguls of India, the Aztecs, and the crowned heads of Europe among others. However, the origin of most well-known emeralds has been obscured by the long shadow of history. Gaston Giuliani of the Institut de Recherche pour le Développement and the Centre de Recherches Pétrographiques et Géochimiques-CNRS in Vandoeuvre, France, Marc Chaussidon of the CRPG-CNRS, and their French and Columbian colleagues sought to clear up this mystery for nine historic emeralds spanning a time period from the Roman occupation of France until the 18th century.

These gems included part of an earring from the Gallo-Roman site of Miribel in France, an emerald from the Holy Crown of France placed there by the crusading Louis IX (St. Louis) in the 1200s, 18th century emeralds from the treasury of the Nizam, princely rulers of the former state of Hyderabad in India, emeralds studied by French founding mineralogist Abbé Hauy for his definitive description of the gemstone in 1806, and an emerald recovered from the famous wreck of the Spanish treasure galleon the Nuestra Señora de Atocha that sank in a hurricane off the coast of Florida in 1622.

The researchers were motivated to take on this project after work in mines in Columbia and Brazil revealed that the deposits from each geographic region--and often from individual mines--are characterized by very specific oxygen isotope levels. Oxygen isotope values in gems such as emeralds reflect the composition and temperature of the fluids that eventually crystallized to form the emerald, as well as the composition and temperature of the rocks that the fluids journeyed through before their consolidation into gemstone. There is a narrow range of these isotope values for each site where emeralds have been discovered worldwide. Along with more traditional gemological aspects, such as optical properties and the inclusion of other materials, researchers can use these unique isotope values to pinpoint where an emerald was "born."

To gather these isotope values, Giuliani, Chaussidon and their colleagues used a technique known as ion microprobe oxygen isotopic analysis. The method works by bombarding the emerald with an electron ion beam, which dislodges oxygen ions from the crystal lattice of the gem that the scientists can collect and analyze. This makes very small craters in the stones that are invisible to the naked eye. The virtually non-destructive method allowed the research team to reassure the curators of these priceless gems that their investigations would not cause irreparable harm, Giuliani said.

The isotope values of the nine emeralds in their study offer an unequaled glimpse at the mining and trade of emeralds from antiquity through the Age of Exploration. For instance, Egypt and Austria were thought to be the only sources of gem-quality emeralds in the ancient world, and the researchers did find that the Holy Crown and Hauy emeralds came from mines in these areas. However, they traced the Miribel earring stone to Pakistan, a previously unrecognized source of emeralds in antiquity. And according to the isotope data, one of the stones from the Hyderabad treasury in India originated in Afghanistan. "The deposits in these areas fall along the old Silk Route," Giuliani said, "and it may be that they were mined or collected there as traders passed though, and brought to Rome and France and elsewhere."

Also surprising was the origin of the remaining three emeralds from the Hyderabad treasury, often known as "old mine" emeralds for their supposed source in lost mines of Asia. Some scholars believe that these emeralds may have made their historical debut as far back in time as the days of Alexander the Great (around 300 B.C.), eventually finding their way into the opulent coffers of the Nizam by the 1800s.

Instead, the isotope analysis showed that these prized stones came from three separate mines in Columbia, indicating that the trade in emeralds from the New World made rapid and significant inroads into Europe and the Near East. Analysis of the emerald from the wreck of the Nuestra Señora de Atocha reveals a Columbian origin for that gem as well, providing further direct evidence of the Spanish role in bringing New World emeralds into Old World trade.

The researchers speculate that many of the emeralds held in large museum caches of treasure in modern-day India may come from Columbian mines, as well as from old Asian sources such as Afghanistan and Pakistan. "Columbian emerald deposits are unique in the world, producing stones with richer color, clarity, and bigger crystals than most emerald deposits," Giuliani said. "We imagine that these were the qualities that the Spanish, and the rest of the world, were interested in."

The researchers plan to apply the oxygen isotope technique to studying rubies next, after completing the necessary survey of the geology and geochemistry of ruby deposits around the world. "This type of analysis was a small part of our work initially," said Giuliani. "But we see that as geologists working with gemstones and gemologists, we may be permitted to make a contribution to the study of trade and human history."