How diamonds form still isn't entirely understood, but laboratory experiments show that the gemstones crystallize only under extreme pressures. Most naturally occurring stones have been traced to the upper mantle, at depths between 93 and 186 miles (150 to 300 km), where pressures can reach beyond 20,000 atmospheres.

For a long time, this put diamonds in competition with a gem called peridot for the title of deepest-occurring gemstone. Peridot is the gem form of a mineral called olivine that makes up more than half of the upper mantle, which extends from the base of the crust down to 255 miles (410 km). But in 2016, scientists described a collection of superdeep diamonds sourced from around 410 miles (660 km), and another batch in 2021 was determined to come from a depth of 466 miles (750 km).