Sapphires form inside volcanoes, not deep in the mantle

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.

- STEPHANIE PAPPAS

But the latest research finds that, instead, sapphires are born higher in the crust, in the hearts of volcanoes where magma rises to only about three miles below the surface. "We can pinpoint this region as the 'crucible' where sapphire formed," said Axel Schmitt, a geologist at Curtin University in Australia, who conducted the work while at Heidelberg University in Germany.