Carbon atoms occur in three different masses, or isotopes.
Virtually all carbon atoms, the ones in a diamond or a tree or you, came from the stars. The narrow range of 13C values for harzburgitic diamonds in the histogram on the top resembles the range of average Earth, indicating that the mantle is the likely carbon source. These data support recycling of once-living carbon from Earth's surface deep into the mantle to form diamond. When ocean floor slides into the mantle, the basaltic rock becomes eclogite, and organic carbon in sediments may become diamond.