Diamonds are crystals made entirely of carbon atoms and are formed in the superior layers of the terrestrial mantle, at depths between 150 and 300 km, even 650 km for blue diamonds, at temperatures of over 1350 degrease and pressures of over 50.000 atmospheres.
Diamonds are, on average, an incredible one billion years old, much older than dinosaurs, which appeared about 250 million years ago and that went extinct 185 million years later. About 450 billion years ago, plants began to expand, and their rests started the formation of petrol and coal, so the carbon needed for the diamonds growth could not have come from coal, but from older melted minerals from the mantle, where today there may still be an immense quantity of diamonds.
In case diamonds are slowly brought to the surface, the lower temperature and pressure transition, will transform them in graphite, so ancient, very rapid and deep volcanic eruptions, brought todays diamond near the surface in a magmatic rock called Kymberlite and those primary deposits are found only in very geological old and stable places called cratons.