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(CNN)Scientists say they have found a trace of ancient life inside a 2.5 billion-year-old ruby.
The ruby sample from Greenland, where the oldest known deposits of rubies are found, contained graphite -- a mineral made of pure carbon. Chemical signatures in the carbon suggested it was a residue of early life.
"The graphite inside this ruby is really unique. It's the first time we've seen evidence of ancient life in ruby-bearing rocks," said Chris Yakymchuk, professor of earth and environmental sciences at the University of Waterloo in Canada, in a news release.
The graphite is found in rocks older than 2.5 billion years ago, a time on Earth when oxygen was wanting in the atmosphere and single-cell life existed only in microorganisms and algae.
To determine whether the carbon was biological in origin, the researchers looked at its chemistry -- specifically the composition of isotopes in the carbon atoms.
"Living matter preferentially consists of the lighter carbon atoms because they take less energy to incorporate into cells," Yakymchuk said. "Based on the increased amount of carbon-12 in this graphite, we concluded that the carbon atoms were once ancient life, most likely dead microorganisms such as cyanobacteria."
The scientists found the rock in Greenland while studying the geology of rubies to better understand the conditions necessary for their formation.
Rubies are a red-colored variety of the mineral corundum.
Sapphires are formed from the same substance. In rubies, chromium produces the distinctive color, while traces of iron, titanium and nickel produce different colored sapphires, including the blue hue usually associated with the gemstone.
The team also found that the graphite likely changed the chemistry of the surrounding rocks to create favorable conditions for ruby growth.
"The presence of graphite also gives us more clues to determine how rubies formed at this location, something that is impossible to do directly based on a ruby's colour and chemical composition," Yakymchuk said in the statement.
The research was published in Ore Geology Reviews last week.