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Diamonds hold a special place in our imagination: they’re valuable, extremely hard, and sometimes just really pretty. Chemically speaking, diamonds are simple – they’re carbon.

Carbon is an important element. All living things are made of compounds that contain carbon. One property of carbon is its ability to form allotropes. Allotropes contain the same element, but their atoms are arranged in different ways. Most elements only form a small number of allotropes – carbon can form many.

When carbon atoms are arranged in sheets, they form graphite, which is used in pencils. Carbon can also form spherical shapes that look like soccer balls – these are called buckyballs. When carbon atoms are arranged in a network, they form diamond. Most diamonds are not completely pure carbon – they often contain impurities, such as oxygen, sulfur and other minerals.

While allotropes contain atoms of the same element, individual atoms in the allotrope can have different numbers of neutrons in their nucleus. Atoms that have the same number of protons, but different numbers of neutrons are called isotopes. By far the most common isotope of carbon has six neutrons, but it can have more or less.

Diamonds form when carbon is subjected to extremely high temperature and pressure. The specific process for forming one type of diamonds, called eclogitic diamonds, is the subject of debate.

A group of researchers measured the ratio of different carbon isotopes in diamond samples, as well the ratio of different oxygen isotopes in impurities in the same samples. They argue that the measured ratios indicate a biological origin for the carbon in eclogitic diamonds, instead of a geological origin. They propose that organic material at the bottom of the ocean was drawn beneath the crust (or ‘subducted’), where temperature and pressure converted it into diamonds.

Chemically they might seem simple, but diamonds and how they form are still the subject of healthy scientific debate.