An interesting article found at these links:
Ring brings ancient Viking, Islamic civilizations closer together
More than a century after its discovery in a ninth century woman’s grave, an engraved ring has revealed evidence of close contacts between Viking Age Scandinavians and the Islamic world.
Excavators of a Viking trading center in Sweden called Birka recovered the silver ring in the late 1800s. Until now, it was thought that it featured a violet amethyst engraved with Arabic-looking characters. But closer inspection with a scanning electron microscope revealed that the presumed amethyst is colored glass (an exotic material at the time), say biophysicist Sebastian Wärmländer of Stockholm University and his colleagues.
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