Early in 2012, someone in Southern Morocco picked up 35 greenish stones, which is believed to be fragments of a meteorite from Mercury.
Purchased by a dealer in Erfoud, Morocco, it was then resold to Stefan Ralew, a meteorite collector from Berlin.
The wrinkled glassy coating on one face of the rock was clearly a fusion crust, a kind of glaze that forms when a meteorite is heated as it passes through the atmosphere.
Looking at other faces he would have recognized it as a type of meteorite called an achondrite, Randy Korotev, WUSTL?s meteorite expert said. That meant it was an exceptional stone.
Most meteorites are stony and of the stony meteorites, almost all (90 percent) are what are called ordinary chondrites. These are pieces of small, unmelted asteroids that are uniform in composition throughout.
About half of the achondrites come from the large asteroid 4 Vesta. Others come from Mars, the Moon, or other asteroids.
To answer the question of origin, the stone's chemistry had to be analyzed.
Both the iron/manganese ratio of an asteroid and the ratios of its oxygen isotopes (variants of the oxygen atom) are thought to serve as 'fingerprints' of its body of origin.
Now, officially designated Northwest Africa 7325 (NWA 7325), the stone had highly unusual chemistry.
What's more, the chemistry was suspiciously similar to that measured by NASA's Messenger probe, which is currently surveying the surface of Mercury from orbit.
"It is high in magnesium and very low in iron, which is what they?re seeing on the surface of Mercury," Korotev said.
"But it's got more plagioclase (an aluminum containing mineral) than they're seeing on the surface of Mercury and it plots funny in 'oxygen isotope space.' It's plotting in a region of oxygen isotope space where we've never had meteorite data points before -- except for a few ureilites, which also have oddball chemistry," he said.
Some chemical ratios didn't match, but that might be because the stone had been 'excavated from depth,' that is blasted into space by a collision that left a deep scar in Mercury.