Every museum and university geology department gets the same enquiry several times a month. Someone has found a heavy, dark, oddly shaped rock and wants to know if it fell from space. The overwhelming majority of the time the answer is no. Rocks that get mistaken for meteorites are common enough to have a nickname among geologists: meteorwrongs.
What a meteorite actually is
Three broad classes. Stony meteorites, about 94 percent of falls, mostly chondrites, which contain millimetre-scale spherical chondrules that formed in the solar nebula before the planets existed. Iron meteorites, fragments of the metallic cores of shattered planetesimals. Stony-iron meteorites, the rarest, including pallasites with olivine crystals suspended in a nickel-iron matrix.
Chondrites are the oldest solid material available to study, at roughly 4.56 billion years, older than any rock on Earth’s surface by a wide margin, because Earth recycles its crust through the process described in the rock cycle.
The tests, in order of usefulness
Fusion crust. A fresh meteorite has a thin, dark, matte-to-glossy rind produced by melting during atmospheric entry. It is usually under a millimetre thick and looks different from the interior. Weathering destroys it over decades, so its absence proves nothing, but its presence is a strong signal.
Regmaglypts. Shallow thumbprint-like depressions across the surface, produced by ablation in flight. They look like someone pressed a thumb into soft clay. Very characteristic, rarely mimicked.
Density. Most meteorites are noticeably heavier than a terrestrial rock of the same size because of their metal content. This is where the specific gravity instinct earns its keep.
The streak test. This one eliminates most false positives. Rub the specimen on unglazed ceramic. Magnetite leaves a black streak. Hematite leaves a red-brown streak. A genuine meteorite leaves essentially nothing, because metallic iron does not produce a coloured powder. Given that magnetite and hematite are the two most common meteorwrongs, this simple test settles the majority of cases. Method in the streak test post.
Nickel. The definitive chemical marker. Terrestrial native iron is vanishingly rare and low in nickel; meteoritic iron typically runs 5 to 20 percent nickel. This requires a lab.
Widmanstätten pattern. Cut and etch an iron meteorite with acid and an interlocking geometric pattern of kamacite and taenite bands appears. It forms only through cooling at a few degrees per million years, which is physically impossible to fake and impossible to produce on Earth.
What fools people
Magnetite and hematite, because they are heavy and magnetic. Industrial slag, because it is dark, heavy, vesicular and often magnetic. Iron-rich concretions, which come in convincingly spacelike shapes. The magnet test alone is close to worthless, since it passes all of these.
Impact glass is a different thing
Worth being clear about a distinction that gets blurred. A meteorite is surviving material from the object itself. Impact glass is terrestrial rock melted by the energy of an impact and is not extraterrestrial material at all.
Libyan desert glass, which we carry as a three-piece lot and as a larger single piece, is the best-known example: nearly pure silica glass scattered across the Great Sand Sea, formed around 29 million years ago by an event still argued over. Its story is in the stone that fell from the sky. It came from an impact. It did not come from space.
If you think you have one
Do the streak test first, it costs nothing. If it passes, photograph it with a scale, note exactly where it was found, and contact a university geology department or a national meteorite collection. Do not cut or clean it. Finding a real one is uncommon but not impossible, and location data is part of what makes it scientifically valuable, the same argument made in provenance and paperwork.
