Gemstones carry the tale of their geographic origins
Spectroscopy techniques allow us to see way beneath the shiny surface of gemstones.
For thousands of years, gemstones have been valued and traded around the world. Along with their beauty, these strong and rare minerals carry with them a unique elemental composition and atomic orientation. These traits act as fingerprints that can show researchers a gem’s past—including where they originated and traveled.
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In a study published August 1 in the journal AIP Advances, a team of scientists used spectroscopic techniques to compare sets of gems and pinpoint where they originated and how they passed through trade routes. They looked at gems found in the Arabian-Nubian Shield—an exposure of mineral deposits near the Red Sea in present day Egypt and Saudi Arabia.
Spectroscopy helped the team identify specific elements inside the gems, including their chemical makeup and structure. These traits influence the gems’ color, differentiated the stones found outside of the Arabian-Nubian Shield, and show which gems are natural versus synthetic.
“We showed the main spectroscopic characteristics of gemstones from these Middle East localities to distinguish them from their counterparts in other world localities,” Adel Surour, co-author and meteorologist and geologist at Galala University in Egypt, said in a statement. “This includes a variety of silicate gems such as emerald from the ancient Cleopatra’s mines in Egypt, in addition to amethyst, peridot, and amazonite from other historical sites, which mostly date to the Roman times.”
A technique called laser-induced breakdown spectroscopy (LIBS) can quickly characterize the gem’s chemical composition, while another called fourier transform infrared spectroscopy (FTIR) determines the functional groups connected to the gem’s structure and detects the presence of water and other hydrocarbons. Raman spectroscopy shows the unique crystalline structure of the gems’ atoms, even for chemically identical materials.
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The team found that iron content likely correlates to the signature purple hue in amethysts and elements including chromium, vanadium, and copper also impact a gem’s color. Synthetic gems that are less expensive and used in lab experiments are exposed by a signature water peak.
The unique crystalline structure of the gems differentiated amazonite beads from Mexico, Jordan, and Egypt, making it possible to follow where they traveled.
“Gemstones such as emerald and peridot have been mined since antiquity,” Surour said. “Sometimes, some gemstones were brought by sailors and traders to their homelands. For example, royal crowns in Europe are decorated with peculiar gemstones that originate from either Africa or Asia. We need to have precise methods to distinguish the source of a gemstone and trace ancient trade routes in order to have correct information about the original place from which it was mined.”
