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Dazzling blue sapphires may look like pieces of heaven brought to Earth, but new research shows these gems come from a different borderland: the boundary between the Earth’s crust and the magma welling up from the mantle, the middle layer of the Earth.
Sapphires are thought to form in the mantle itself or in the lower parts of the crust, the study’s lead author said Axel Schmitta geologist at Curtin University in Australia who conducted the work while at Heidelberg University in Germany, told Live Science. But the new research shows that sapphires are instead born higher in the crust, at the hearts of volcanoes where magma rises to only about 3 miles (5 kilometers) below the surface.
“We can identify this area as the ‘crucible’ where sapphire was formed,” Schmitt said.
Gem-quality sapphires typically come from placer deposits, river sediments that wash minerals from their original source rocks. Without those source rocks, it’s difficult to get information about how the gems formed. Schmitt and his colleagues focused on the Eifel Formation in western Germany, which was formed by volcanoes over a long period of time that stretched from the Cretaceous period (145 million to 65 million years ago) to the most recent eruption 13,000 years ago.
“The Eifel volcanic field shows many similarities to 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 of sapphires found there.
Sapphires are made of the mineral corundum, which is mostly pure aluminum and oxygen. But the stones also contain imperfections called inclusions, which are incorporated into the gemstone as it forms. Using ratios of radioactive uranium and lead in these inclusions, the researchers determined the sapphires’ ages. The team also looked at different versions of oxygen in the sapphires, which reveal where the sapphires’ chemical building blocks came from. They studied 223 tiny sapphire grains, none of which were gem-quality.
Together, these trace elements revealed that the sapphires formed less than 2.5 million years ago, meaning they formed at the tail end of the volcanic activity that formed the Eifelveld. Their oxygen molecules were similar to those in magmas brought to the surface, Schmitt said. This meant the sapphires formed at the boundary between surface rock — the crust — and magma.
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“The likely environment for this is the edge of a subsurface magma intrusion, where the surrounding rocks are heated, melted and mixed by contact with magma,” Schmitt said.
Because the researchers knew the depth of the magma in the Eifelveld, they were able to pinpoint this formation depth at between 3.1 and 4.3 miles (5 and 7 km). Schmitt’s next goal is to find mineral “fingerprints” that will help determine the origins of sapphires as a way to trace gemstone supply chains and ensure ethical business practices, especially since many sapphires are mined in developing countries where environmental regulations and human rights protections are not always strict.
While sapphires are not known to fuel and finance conflict in the same way as “blood diamonds,” There are occasional allegations of human rights abuses by gemstone mining companies.
