The Hope Diamond is one of the world’s most famous jewels. It was in the possession of a series of people: kings, bankers, rich women and thieves, before its arrival 60 years ago at the Smithsonian Institution in Washington, D.C. A rare blue diamond, it has a complex social history, to be sure. But, far more so is its geological history, researchers say in a new report. “This is the first time anyone has come up with a fact-based study or model for how blue diamonds form,” said Gemological Institute of America research scientist Evan Smith. He led the study published in the journal Nature. The group looked at 42 blue diamonds, including one from South Africa that recently sold for $25 million in 2016. Researchers could tell where the stones were formed based on the very small minerals trapped inside. The scientists found the gems can form at least 660 kilometers below the earth’s surface, in a part called the lower mantle.