November Editorial      

    Fingerprinting Emeralds

      When a method of identifying the origin of emeralds was announced four years ago it made headlines. Giuliani, Chaussidon and their colleagues at the National Centre for Scientific Research (CNRS) in Nancy, France used a technique known as ion microprobe oxygen isotopic analysis. This works by bombarding an emerald with an electron ion beam, which dislodges oxygen ions from the crystal lattice of the gem for scientists to collect and analyze. The process makes very small craters in the stones that are invisible to the naked eye. 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 originally crystallized to form the emerald, as well as the composition and temperature of the rocks that these fluids journeyed through before their crystallization. There is a narrow range of these isotope values for each site where emeralds have been discovered, so a study of the isotope values in an emerald reveals its place of origin. The technique has proven to be extremely precise, much more so than previous methods.

      Giuliani and his colleagues turned their attention to some historically valuable emeralds of unknown or disputed origin. They analysed an emerald in an earring discovered at the ancient Gallo-Roman site at Miribel in France; an emerald placed in the Holy Crown of France 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; and an emerald recovered from the famous wreck of the Spanish treasure galleon, the Nuestra Seņora de Atocha, which sank in a hurricane off the coast of Florida in 1622. Their findings included some surprises. Until then, Egypt and the area of modern Austria were thought to be the only sources of gem-quality emeralds in the ancient world. The researchers found that the Holy Crown and Hauy emeralds also came from mines in these areas. However, the Miribel earring stone was traced to Pakistan, previously unknown as a source of emeralds in antiquity. 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 there or collected as traders passed though, and brought to Rome and France and elsewhere." The emerald from the wreck of the Nuestra Seņora de Atocha was from where Columbia is today.

      Four years on and Alain Cheilletz, Philippe de Donato and Odile Barres at the National Polytechnic Institute of Lorraine (INPL) in Nancy and CNRS have unveiled a new approach. This method is based on infrared spectra. There are lines in emerald spectra that correspond to deuterated water, in which one of the hydrogen atoms of the water molecule is replaced by its isotope deuterium. Atoms of different rare-earth elements strain the water bonds, producing up to five additional bands in the pattern. The main advantage of studying spectra over ion microprobe oxygen isotopic analysis is that the new technique is is non-destructive and more importantly relies on equipment that gem labs use routinely. The former method required specialist equipment and vaporised a tiny amount of the gem, which was not popular with either gem dealers or labs.

      The group lead by Alain Cheilletz collected samples from 46 emerald mines worldwide and found that their spectra fell into five different groups that corresponded to geographical regions. Even within a group there were differences between individual mines. If this method lives up to its early promise it will be extremely useful not only for fingerprinting famous emeralds but also in beating the illegal emerald trade.

      Unlike rubies and diamonds, emeralds are not normally considered "conflict" gemstones (jewels that help to fund the illegal purchase of weapons). However, the emerald trade has been linked to illegal drug trafficking and paramilitary groups in Colombia and in other troubled regions where the gems are mined, including Afghanistan.