Diamonds form deep in the mantle, where they mostly remain unless propelled to the surface in a volcanic eruption. Diamond is by far our most important mineral messenger for discovering the history and chemistry of Earth’s convecting mantle. Diamond provides a window to otherwise inaccessible geological processes that churned away 100 kilometres underground and often occurred billions of years ago.1 Chemically, diamond is exceptionally pure: 99.9 percent or more elemental carbon. Diamond lasts almost forever, being the hardest known natural material by quite a margin. And since 1948, De Beers has used the marketing tagline “A diamond is forever” continuously to promote its diamond engagement rings.