Earth’s mantle, some 1,800 miles thick, begins about 25 miles down and is sandwiched between the thin outer crust and the dense, iron core. This ancient rock stuffing makes up more than three-quarters of the planet’s volume. But because it is inaccessible, scientists know little about it.

Seismic studies have revealed how a convection current within the mantle allows its contents to circulate. Ocean island basalts — rocks that cooled after volcanic eruptions of magma from the mantle — have offered hints of the mantle’s chemical composition. But neither source has revealed what the mantle is made of at any particular depth.

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That is partly because basalt is a cooled mixture of elements collected during its molten travels. It cannot be parsed. But a diamond does not change physically or chemically on its journey, and whatever it collects can be studied in isolation.

Diamonds form at different depths. As they do so, they trap minerals and fluids that contain carbon, water and noble gasses, capturing snapshots of those environments. These “inclusions” are the flaws that make diamonds cloudy. For earth scientists, they are precious — microscopic parcels of ancient elements from specific times and places, perfectly preserved and protected.