Meteorites are natural fragments of rock or metal that originate in space, survive their fiery descent through Earth’s atmosphere, and land on the ground. These celestial visitors are far rarer than common terrestrial rocks. Identifying a genuine space rock requires inspecting its unique characteristics, which result from its cosmic journey and composition. These distinct features offer a practical guide for visual identification.
The Signature External Appearance
The intense friction generated during atmospheric entry creates the fusion crust, a characteristic outer layer formed when the rock’s surface melts and quickly solidifies. This results in a thin, dark coating, often less than two millimeters thick. A freshly fallen specimen displays a jet-black, glassy, or matte surface, which may show subtle flow lines or tiny contraction cracks.
During descent, atmospheric pressure and ablation sculpt the rock’s surface. This process often leaves shallow, thumbprint-like indentations called regmaglypts, which strongly indicate a meteorite’s identity. The shape is usually irregular but rarely sharp-edged, as intense heat tends to round off corners and protrusions.
As a meteorite remains on Earth, the fusion crust begins to weather and oxidize, causing the initial black color to fade. The surface transitions to a rusty, reddish-brown color due to the iron content reacting with moisture and oxygen. Remnants of the original melted layer are often still visible, contrasting with the interior material.
Weight, Density, and Magnetic Testing
A fundamental property of most meteorites is their high density, making them feel notably heavy for their size compared to common Earth rocks. This elevated mass is primarily due to the presence of metallic iron and nickel. For instance, the most common stony meteorites, called chondrites, often possess a density about one and a half times that of typical Earth rocks.
The metallic content allows for a simple field test: magnetism. Nearly all meteorites, including iron meteorites and the majority of stony meteorites, contain enough nickel-iron metal to be strongly attracted to a magnet. Iron meteorites will cling tenaciously, while stony meteorites might require a strong neodymium magnet for clear attraction.
A weak attraction can be tested by suspending a magnet from a string and observing if the rock influences the magnet’s swing. While rare achondrites may contain little metal, a positive magnetic reaction is a powerful initial sign.
What the Interior Reveals
To examine the interior, a small corner of the suspected specimen must be chipped or ground away to create a fresh surface. The exposed interior of a stony meteorite often reveals small, spherical mineral grains known as chondrules. These structures are unique to space rocks that have not undergone extensive melting and are not found in terrestrial rocks.
Chondrules typically measure less than a millimeter in diameter and appear as tiny, distinct spheres embedded in a finer-grained matrix. The interior of most meteorites also shows visible flecks of shiny, silver-colored nickel-iron alloy, which stand out brightly.
If the meteorite has been exposed to the elements for a long time, the internal metal grains will have begun to rust. This oxidation causes a rusty-red or brownish stain to permeate the interior rock matrix. The contrast between the dark fusion crust and the lighter, speckled interior is a definitive characteristic.
Avoiding Meteorite Look-Alikes
The search for meteorites is often complicated by common Earth materials that share some of their characteristics, frequently termed “meteorwrongs.” Industrial slag, a byproduct of metal smelting, is one of the most common mimics, sometimes appearing melted, dense, and even magnetic due to its iron oxide content.
Slag, however, almost always contains small bubbles or holes called vesicles, which are formed by escaping gas during cooling. True meteorites rarely contain these vesicles, making their absence a reliable way to rule out slag.
Other common look-alikes are the terrestrial iron oxides magnetite and hematite, which are also dense and magnetic. These can be differentiated using a streak test, where the rock is scraped across an unglazed ceramic surface.
Hematite will consistently leave a reddish-brown streak, and magnetite will leave a black or gray streak. Most meteorites, even those with a dark exterior, will leave little to no streak on the ceramic, providing a simple final check to eliminate common magnetic terrestrial rocks.