Diamonds come from deep inside the earth, where high temperatures and high pressure naturally exist. Most were formed from carbon millions of years ago, which has been in the earth’s mantle since its origins. Some diamonds, however, are made from carbon that comes from the bodies and shells of prehistoric micro-organisms. Their main deposits consist of diamondiferous pipes, or tubes of mineral-rich volcanic rock. These rise to the surface from their primary deposits through volcanic activity in magma or molten rock, which pushes the kimberlite, a volcanic diamond-bearing rock, upwards.