Study uncovers deep-Earth origins of world's largest diamonds

A new study led by South Africa's University of Cape Town (UCT) has uncovered fresh clues about the origin of some of the world's largest and most valuable diamonds, linking their formation to unusual iron-rich regions deep within the Earth's mantle.

According to a statement released by the UCT on Monday, the research, conducted by the Kimberlite Research Group in the Department of Geological Sciences at the university, was published in the esteemed journal Nature Communications in April 2026.

It focused on a rare category of gem-quality diamonds known as CLIPPIRs – Cullinan-like, large, inclusion-poor, pure, irregular, resorbed diamonds – whose formation conditions had long remained poorly understood.

According to the study, kimberlite rocks that transported these diamonds to the surface consistently sampled anomalous iron-rich domains located more than 150 km beneath the Earth's surface at the base of the lithosphere.

The team found that these domains carried distinctive isotopic signatures pointing to ancient oceanic crust that was subducted and later incorporated into the deep mantle.

The study was conducted in collaboration with researchers from the Carnegie Institution for Science in Washington and the China University of Geosciences in Beijing.

Associate Professor Geoffrey Howarth, lead author of the study, said interactions between rising kimberlitic melts and these deep iron-rich regions produced the large mineral crystals characteristic of CLIPPIR-bearing rocks.

"These extraordinary diamonds – some of the largest and most valuable gems on Earth – have long been a mystery. Our study shows that they grew in an unusual iron-rich environment deep beneath the continents, formed from ancient oceanic crust that was dragged down by subduction and then accreted at the base of the lithosphere," said Howarth.

The findings also indicate that such iron-rich, isotopically anomalous domains are a widespread and important source of geochemical heterogeneity in volcanic rocks erupted across the planet.

"By reading the chemical fingerprints preserved in the mineral olivine brought up by kimberlite eruptions, we can now trace where these exceptional diamonds come from and how to find more of them," Howarth added.

The Cullinan Diamond is the largest diamond ever found, weighing 3,106 carats when discovered in 1905 near Pretoria, South Africa.