Rubies are among the most popular jewelry gemstones. They are made of a mineral called corundum, or aluminum oxide, and contain traces of other substances. One is the element chromium, that gives the ruby its red color. In 2021 an international research team discovered other traces, linking rubies to early life on Earth. The researchers were studying two-and-a-half billion-year-old rubies from a rock formation in Greenland, to learn how rubies form. They were surprised to find the rubies contained traces of graphite. Graphite is the material ‘lead’ in pencils is made of. It is a mineral of pure carbon.
Carbon atoms come in several forms, called isotopes, which differ in atomic mass. Most have an atomic mass of twelve. But some have one or two extra neutrons, giving them an atomic mass of thirteen or fourteen. The chemistry of living things favors the version of carbon with an atomic mass of twelve. So, carbon that comes from the remains of living things has a higher fraction of carbon twelve compared to carbon thirteen and fourteen than carbon that comes from things that were never alive. The researchers found that the graphite in the rubies had a high fraction of carbon twelve, indicating it probably came from the dead remains of living organisms.
Two and a half billion years ago, Earth was a hostile planet. The only living things known to have existed were simple microorganisms. The discovery that traces of ancient life might survive in rubies provides a new window into that world. Similar methods may help us find evidence of past microbial life on Mars.