How to distinguish meteorites from ordinary rocks

Meteorites are often particularly heavy, magnetic, and bear characteristic marks from being heated as they plunged into the atmosphere.

Close-up of Hoba, one of the largest meteorites ever discovered. (Photo: Travel Telly).

Meteorites typically originate from asteroids or comets, travel through the atmosphere, and land on Earth. There are three main types of meteorites: iron, stony-iron, and stony. Each type can be divided into smaller groups based on mineral content, structure, and chemical composition. Some types of meteorites, such as chondrites, can be more than 4.5 billion years old and are extremely rare, but more common types, such as iron meteorites, occur quite frequently.

So how do you tell a meteorite from a regular rock? One way to test it is to measure its density . Meteorites contain iron and other dense materials, which means they will be much heavier than regular rocks of the same size.

Meteorites that contain a lot of iron are magnetic and will often attract a magnet. However, this is not a hard and fast rule, as some rare meteorites are not magnetic.

Unlike normal rocks that come from Earth, meteorites take on many odd shapes after being heated by the atmosphere. They can even bear unusual indentations on their surfaces called regmaglypts , which look a bit like a potter's fingerprints in wet clay. These indentations appear when the meteorite's outer layer melts as it descends to Earth's surface.

This process can even create a coating on the surface of the meteorite called a fusion crust , which looks like a black eggshell. The surface of a fresh meteorite is usually shiny due to the impact of passing through Earth's atmosphere at high speed. If it has been falling for a long time, the meteorite can take on a brownish color as the iron begins to rust.

In addition, flow trails - ultra-thin lines formed by molten meteorites entering the atmosphere, which can be even smaller and thinner than a human hair - are also a feature to identify meteorites.

Flow streaks on the iron meteorite Taza (NWA 859). (Photo: Geoff Notkin)

In addition to the magnetic test , the person who finds a suspicious stone can also perform a scratch test . Meteorites that leave no streaks after being rubbed on an unglazed ceramic surface. Stones that leave black or red streaks are likely to contain magnetite or hematite, minerals not normally found in meteorites. However, this method is not completely accurate, as some stones do not leave streaks.

If the finder has the means, he or she could punch a hole in the specimen and see if there are any shiny metal fragments, which could be a sign that the stone came from outer space.

One can also use the process of elimination to distinguish meteorites from ordinary rocks. Meteorites do not contain crystals like quartz. Most quartz forms from cooled magma, not from space.

Volcanic rocks sometimes contain bubbles from this cooling process, but meteorites do not contain bubbles. Likewise, meteorites typically do not contain vesicles – tiny voids found in volcanic rocks like pumice.

Laboratory testing is the way to get a more definitive answer about meteorites. Man-made iron from smelting processes (called slag) is sometimes mistaken for meteorites, but this confusion can be avoided by testing for the presence of nickel. Earthly iron is typically nickel-free, while meteoritic iron from outer space contains at least trace amounts of pure nickel.