The conditions for diamond formation to happen in the lithospheric mantle occur at considerable depth corresponding to the requirements of temperature and pressure. These depths are estimated between 140 and 190 km though occasionally diamonds have crystallized at depths about 300 km as well.
The rate at which temperature changes with incresing depth into the Earth varies greatly in different parts of the Earth. In particular, under oceanic plates the temperature rises more quickly with depth, beyond the range required for diamond formation at the depth required. The correct combination of temperature and pressure is only found in the thick, ancient, and stable parts of continental plates where regions of lithosphere known as (Cratons) exist. Long residence in the cratonic lithosphere allows diamond crystals to grow larger.
Through studies of carbon Isotope ratios (similar to the methodology used in carbon dating, except with the stable isotopes C-12 and C-13, it has been shown that the carbon found in diamonds comes from both inorganic and organic sources. Some diamonds, known as harzburgitic, are formed from inorganic carbon originally found deep in the Earth’s mantle. In contrast, eclogitic diamonds contain organic carbon from organic detritus that has been pushed down from the surface of the Earth’s crust through subduction before transforming into diamond.
Diamonds that have come to the Earth’s surface are generally quite old, ranging from under 1 Billion to 3.3 billion years old. This is 22% to 73% of the age of the Earth.