In 1996, scientists discovered what may be the strangest stone ever found, in an equally strange section of the Sahara desert that's littered with unique yellow glass. Nicknamed the Hypatia stone, the relic was later found to be extraterrestrial in origin, but was unlike any known kind of meteorite or comet. A new study has deepened the mystery even further, finding that Hypatia could predate the formation of the Solar System, or have interstellar origins.

A few years ago, a University of Johannesburg team declared the Hypatia stone to be the first "comet nucleus" – the rocky, solid center of a comet – found on Earth. The idea supports the accepted story behind the Libyan Desert Glass Field where it was discovered, since the explosion from the impact would have flash-fried the sand and created the yellow silica glass dotting the area.

But the Hypatia stone's origin story isn't quite that simple. The Johannesburg team has now analyzed its mineral composition and found that although it's definitely a space rock, it doesn't follow the basic recipe common to everything else in the Solar System.

Billions of years ago, the Solar System began life as a huge cloud of dust and gas surrounding the Sun. Over time, this material clumped together to form the planets and dwarf planets, with the leftover fragments drifting around as asteroids and comets. As such, these wayward travelers are generally made of the same stuff as Earth. But that's not the case with Hypatia.

"If it were possible to grind up the entire planet Earth to dust in a huge mortar and pestle, we would get dust with on average a similar chemical composition as chondritic (rocky) meteorites," says Jan Kramers, lead researcher on the study. "In chondritic meteorites, we expect to see a small amount of carbon and a good amount of silicon. But Hypatia's matrix has a massive amount of carbon and an unusually small amount of silicon.

"Even more unusual, the matrix contains a high amount of very specific carbon compounds, called polyaromatic hydrocarbons (PAH), a major component of interstellar dust, which existed even before our solar system was formed. Interstellar dust is also found in comets and meteorites that have not been heated up for a prolonged period in their history."

The PAH in the Hypatia matrix is probably responsible for preserving the stone long enough to be studied, the team says. The impact of the space rock was forceful enough to convert the carbon-rich material into a layer of micro-scale diamonds, protecting it against millions of years of weather.

Dotted throughout Hypatia's rocky body are metallic "inclusions," and on closer inspection these turned up even more quirks. The researchers analyzed the mineral grains in these inclusions and found some compositions that have never been seen before, on Earth or otherwise.

"The aluminum occurs in pure metallic form, on its own, not in a chemical compound with other elements," explains Georgy Belyanin, first author of the study. "This occurrence is extremely rare on Earth and the rest of our solar system, as far as is known in science. We also found silver iodine phosphide and moissanite (silicon carbide) grains, again in highly unexpected forms. The grains are the first documented to be found in situ without having to first dissolve the surrounding rock with acid. There are also grains of a compound consisting of mainly nickel and phosphorus, with very little iron; a mineral composition never observed before on Earth or in meteorites."

All of these anomalies together suggest that Hypatia – or at least parts of it – began forming before the Solar System did. The rocky outer shell, the team says, probably isn't presolar, but that starts to rewrite our understanding of how the Solar System began.

"For starters, there are no silicate minerals in Hypatia's matrix, in contrast to chondritic meteorites (and planets like the Earth, Mars and Venus), where silicates are dominant," says Kramers. "Then there are the exotic mineral inclusions. If Hypatia itself is not presolar, both features indicate that the solar nebula wasn't the same kind of dust everywhere – which starts tugging at the generally accepted view of the formation of our solar system."

One thing that the team says is known for sure is that the strange stone was formed in an extremely cold environment of below -196° C (-321° F). That indicates that Hypatia came from the Kuiper belt beyond Neptune, or even the Oort Cloud, out in interstellar space.

"We know very little about the chemical compositions of space objects out there," says Kramers. "So our next question will dig further into where Hypatia came from."

The research was published in the journal Geochimica et Cosmochimica Acta.