After closely inspecting massive diamonds, scientists suggest they not only have unusual sizes but also unusual origins.

Slivers of embedded iron in these gems, surrounded by special gases, point to a liquid metal origin and also confirm metal-saturated conditions deep in the Earth, the researchers say.

To date, geologists have puzzled over the origin of large diamonds like the famous Cullinan Diamond, discovered in South Africa in 1905, or the widely publicized Lesedi La Rona stone, uncovered in Botswana in 2015.

Historically, research into such diamonds has been nearly impossible due to the high value of the jewels and the fact that they rarely contain inclusions that might shed light on their geological origin.

Here, Evan Smith and colleagues screened thousands of diamonds submitted daily to the grading operations of the Gemological Institute of America, searching for appropriately sized samples with detectable inclusions.

The researchers found metallic iron slivers in more than 30 exceptionally large stones. These were the growth medium for the big diamonds, the authors say, setting these stones apart from other diamonds of lesser size, which form differently.

The metallic inclusions in the large gems were surrounded by reducing gases (those that donate electrons), providing direct evidence of a long-suspected reaction in Earth's mantle that could have precipitated the metals in Earth's outer core.

This result is important for understanding how volatile substances like carbon might cycle through Earth's interior over time.

Research paper