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Chapter 5: Patterns in Nature: Minerals
Feature Articles
The Human Angle: Geologists Take A Closer Look at the Holy Crown of France
by Elizabeth Lane MasonEmeralds are more than just precious gems. Geologists have found that these verdant minerals have an atomic signature that can reveal where on earth they were formed. This information might help historians to sort out ancient trade routes and reveal the murky origins of some famous Old World emeralds. Dealers may be able to use the technique to authenticate the quality of their gems. And, perhaps geologists could discover clues to a long-lost emerald mine.
Emeralds are a type of beryl (Be3Al2(Si6O18)), usually formed as magma cools to become granite. Beryl is typically white or pale green or blue, but if the granite happens to encounter rocks rich in chromium or vanadium as it is cooling, emeralds can form. However, Colombian emeralds, sought after for their intense color and exceptional clarity that sets them apart from most other emeralds, have a peculiar and unique history.
Hundreds of millions of years ago, South American black shales containing chromium and vanadium were washed off the west coast of the continent and collected on the sea floor. Subsequently, the eastward-moving Caribbean Plate collided with Brazilian Plate, forcing the sea floor and its blanket of black shales onto the South American continent. As the plates continued to smash together, folding and faulting provided conduits for hot fluids to rise through the black shales, picking up vanadium, chromium and the other ingredients of emeralds along the way. The mixture collected beneath impermeable shale layers until the pressure became great enough to break the rock apart, squirting the mineral-laden fluid into the cracks where it cooled, crystallizing the dazzling emeralds.
Geologists working to piece together the history of the Colombian emeralds discovered that the minerals have oxygen isotope ratios that are specific to the mine that each mineral came from. Researchers from the Petrographical and Geochemical Research Center in Vandouevre-les-Nancy, France, took this discovery to museums to see if they could use these unique oxygen isotope footprints to uncover the origins of emeralds in ancient artifacts. The results of their work, published in the January 28, 2000 issue of Science, shed light on the origins and history of precious specimens such as a 13th century French crown and the treasure from a sunken Spanish galleon. The scientists are currently working to see if other gems such as rubies and sapphires may have similar oxygen isotope tags.
REFERENCES
Giuliani, G., Chaussidon, M., Schubnel, H-J., et.al., 2000, Oxygen isotopes and emerald trade routes since antiquity: Science, v.287, p. 631-633.
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