As with plenty of other precious gemstones, they contain what are called inclusions—tiny bits of surrounding rocks captured as the diamonds form. Much like how an acid and a base immediately react to form water and a salt, oxidized carbonate minerals and reduced metals don’t coexist for long. But these findings suggest that oxidized rocks occur deeper than that—and thus so might kimberlite rocks. (Another way chemists think diamonds may form is by precipitating out of carbon-rich fluids that cool as they rise upward in the mantle, like sugar crystalizing from syrup. The nickel-rich inclusions might also help explain an odd occurrence in some diamonds: occasional atoms of nickel seem to replace the carbon of these diamonds’ crystal lattice.