Brilliant-blue sapphires look like bits of sky brought down to Earth — but a new study finds these gemstones are from a different boundary: the one between the planet's crust and magma welling up from the mantle, Earth's middle layer. Sapphires have been thought to form in the mantle itself or in the lower sections of the crust, study senior author Axel Schmitt , a geologist at Curtin University in Australia who conducted the work while at Heidelberg University in Germany, told Live Science. But the new research finds that, instead, sapphires are born higher in the crust, in the hearts of volcanoes where magma rises to only about 3 miles (5 kilometers) below the surface.
"We can pinpoint this region as the 'crucible' where sapphire formed," Schmitt said. Gem-quality sapphires typically come from placer deposits, which are river sediments that wash minerals out of their original source rock. Without those source rocks, it's tough to get information on how the gems formed.
Schmitt and his colleagues turned to the Eifel formation in western Germany, which was created by volcanoes over a long period stretching from the Cretaceous period (145 million to 65 million years ago) to the most recent eruption 13,000 years ago. "The Eifel volcanic field shares many similarities with basaltic volcanic fields that are often identified as the sources for sapphire placer deposits," Schmitt said. "However, it is much younger," making it a promising place to investigate the chemistry and age .