The article is shite (science writers--absolutely fucking useless) so I had to read the actual paper. The paper deals with the composition of carbon in the graphite contained in the ruby. Here we are interested in the [C13 to C12 ratio](https://en.wikipedia.org/wiki/%CE%9413C) of the sample, both of which are stable isotopes. The ratio of the two isotopes gives clues to the origin of carbon. Biological processes and some non-biological processes [don't treat all carbon atoms equally](https://en.wikipedia.org/wiki/Kinetic_fractionation). Biological processes have a bias towards the lighter C12. From [the paper](https://www.sciencedirect.com/science/article/pii/S0169136821004431): >Untreated sample aliquots yield δ13C values that vary from –14.2 to –17.2 ‰ whereas the acid-treated samples yielded values ranging from–13.3 to –15.7 ‰. The variability indicates some carbon isotopic heterogeneity in the corundum schist. Nonetheless, these values are within the range of organic carbon (Schidlowski, 1987, Schidlowski, 2001), but, given the potential for carbon isotope fractionation during high-temperature metamorphism, it is unclear if these values represent bona fide organic signatures or fractionation of a non-biogenic source. In other words, the carbon might have biogenic origin, or it could be non-biogenic because some non-biogenic sources can produce these isotope ratios. The article is titled "2.5 billion-year-old traces of life locked inside primeval ruby" when the paper says that while it could be a sign of life, we don't fucking know. What a fucking joke.