Gemstones ‘spill the beans’ on ancient trade routes

Amethyst. (Reference image by Jason Deines, Pexels.)

Modern spectroscopic techniques used to analyze gemstones’ unique characteristics are also helping researchers uncover ancient trade routes.

In a paper published in the journal AIP Advances, scientists at the American Institute of Physics explain how laser-induced breakdown spectroscopy (LIBS), Fourier transform infrared (FTIR) spectroscopy, and Raman spectroscopy, allowed them to identify elements that influence gems’ colour, differentiated stones found within and outside their samples from the Arabian-Nubian Shield, and distinguished natural from synthetic materials.

The Arabian-Nubian Shield is an exposure of mineral deposits that sandwiches the Red Sea in current-day Egypt and Saudi Arabia. The deposits date back to the earth’s earliest geological age, and the precious metals and gemstones have been harvested for thousands of years.

“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 of the paper, said. “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.”

The various spectroscopic techniques Surour and his colleagues employed revealed different information about the stones. LIBS quickly characterizes chemical composition, while FTIR determines functional groups connected to the structure and indicates the presence of water and other hydrocarbons. Even for chemically identical materials, Raman spectroscopy shows the unique crystalline structure of the gems’ atoms.

The authors identified that iron content correlates to amethysts’ signature purple hue, and other elements such as copper, chromium, and vanadium are also responsible for colorization. A signature water peak exposes lab-grown synthetic gems, which are useful for scientific purposes and identical to natural gems but are less expensive.

Crystalline structure differentiated amazonite beads from Mexico, Jordan, and Egypt.

“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.”