Every year, hundreds of people bring us rocks they've found in the desert, wondering if they might be from space. First, we inspect them visually for a few tell-tale signs that they did not form here on Earth:
Weight -- Unusual density is one of meteorites' more characteristic features. Iron meteorites are generally 3.5 times as heavy as Earth rocks of the same size, while stony meteorites are about 1.5 times as heavy. However, iron ores are also exceptionally heavy.
Appearance -- Of all the rocks that fall from the sky, stony meteorites are by far the most common, making up 85-90% of all meteorites. They can be tricky to identify, as they more closely resemble terrestrial rocks than do the iron meteorites. However, if your specimen contains quartz, it is not a meteorite. Quartz is produced on the earth at plate margins; other planetary bodies like asteroids do not have this kind of setting and do not produce quartz crystals.
Meteorites are rarely round or aerodynamically-shaped, and virtually never have a bubbly appearance or small holes in the exterior or interior. The surface of a meteorite is generally black or rusty brown (not shiny silver, unless the fusion crust has completely disintegrated), very smooth and featureless, and has shallow depressions or cavities resembling thumbprints.
Here are some simple tests you can use to determine if a suspected meteorite is worth sending to a testing laboratory for analysis:
- The first test your potential meteorite will have to pass is the magnetic test. 99% of all meteorites are attracted to a strong magnet on a string (including stony meteorites, which contain 3-30% nickel). However, so are metal artifacts and iron ore. This is a simple test that will rule out tektites and many terrestrial rocks. Keep in mind, though, that exposing a meteorite to a magnet can corrupt or change its natural magnetic field -- possibly destroying research information, if your find is important. If you're concerned about this, use a compass needle to determine if your specimen is magnetic.
- Next, the streak test. Iron ore is the most common meteor-wrong; magnetite (lodestone) is very magnetic, and hematite is mildly magnetic. Fortunately, both of these minerals will leave a distinctive mark on a streak plate. Take your suspected space rock and rub it vigorously on the unglazed side of a ceramic tile (or the underside of your toilet tank cover, if you don't have a tile). If it leaves a grey-black streak (like a lead pencil), what you have is almost certainly magnetite; if the streak is red-brown, you likely have hematite. If there is no streak, your specimen has passed the second test!
- Finally, the most complicated and definitive test you can do in your home -- a test for nickel. All iron meteorites and nearly all stony meteorites contain some nickel; a chemical test for nickel is definitive for meteorites 99% of the time. You should be able to buy all the chemicals you need at a hardware store (though you might need to get dimethylglyoxime from an online source). Be careful! Wearing gloves and goggles, dissolve about 1 gram of your suspected meteorite in heated chloridric acid. Add a few drops of nitric acid, then a few drops of citric acid, then add ammonium hydroxide. Filter the solution if it's cloudy. Then add a few drops of dimethylglyoxime. A nice bright cherry red color will indicate the presence of nickel.
If your rock passes all these tests, there's a good possibility you have a meteorite! Contact your local university, or check online for a reputable testing laboratory. Maybe you've found the next Sikhote-Alin, Gibeon, Nantan, Campo del Cielo, or NWA!