During eruption these pipes are open to the surface, resulting in open circulation; many xenoliths of surface rock and even wood and fossils are found in volcanic pipes.
Four characteristics, known informally as the four Cs, are now commonly used as the basic descriptors of diamonds: these are carat (its weight), cut (quality of the cut is graded according to proportions, symmetry and polish), color (how close to white or colorless; for fancy diamonds how intense is its hue), and clarity (how free is it from inclusions). The formation of natural diamond requires very specific conditions—exposure of carbon-bearing materials to high pressure, ranging approximately between 45 and 60 kilobars (4.5 and 6 GPa), but at a comparatively low temperature range between approximately 900 and 1,300 °C (1,650 and 2,370 °F).
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.
This is because cratons are very thick, and their lithospheric mantle extends to great enough depth that diamonds are stable.
Not all pipes contain diamonds, and even fewer contain enough diamonds to make mining economically viable.
) is a metastable allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice.