Credit: Petra Diamonds Limited

Some diamonds are a beautiful blue colour, because they contain trace quantities of boron. But blue diamonds are formed in Earth’s mantle, whereas boron is concentrated in the crust. So where did these diamonds get their boron from? In a paper in Nature, Smith et al. provide the answer to this geochemical conundrum (E. M. Smith et al. Nature 560, 84–87; 2018).

The authors analysed minerals trapped in 46 blue diamonds, and from this worked out that the gemstones must have formed in the lower mantle. Their analysis also suggests that the material from which the diamonds formed contained water and came from the oceanic lithosphere (tectonic plates beneath the sea), a rich source of boron.

The findings mean that blue diamonds are some of the deepest ever found. Moreover, they reveal a geochemical pathway that extends from the oceanic lithosphere at Earth’s surface to the lower mantle, and a potential route for the ultra-deep cycling of water in our planet.