How to Identify a Meteorite Rock and Common Lookalikes

Turn the rock in daylight and look for rounded holes, frothy patches or a sponge-like texture. Abundant gas holes strongly favor volcanic rock or furnace slag. Rare meteorites can contain vesicles. Check the matrix, fusion crust and broken face together before ruling out a specimen.

Slag is the most common meteorwrong. It is glassy, often with a rainbow or metallic sheen, with bubbles of many sizes and a ropy or flowed surface. It turns up along old railway lines, near former foundries and in fill; landscaping lava rock has the same holes with a dull gray or red surface.

A fresh fall has a fusion crust: a thin black rind under a millimeter thick, matte like charcoal on stony meteorites, glossy or lightly ridged on irons. It wraps the whole stone smoothly and stops at any later break, where the lighter interior shows. Old finds weather brown or rusty and the crust may be mostly gone, so a brown rock is not ruled out.

Regmaglypts are shallow, rounded depressions that look like thumbprints pressed into clay, left as the surface melted and streamed away. Weathered Earth rocks show pits too, so look at the edges: regmaglypts are smooth and rounded, dissolution pits in limestone are sharp and angular, and bubbles in slag are round holes with thin glassy rims.

Shape helps a little: irregular with rounded corners, because sharp edges melt first; not a sphere and not a flat river-worn pebble.

Stony meteorites run about 3.3 to 3.6 grams per cubic centimeter and irons about 7.5 to 8, so both feel heavier than a granite or sandstone pebble of the same size. Weigh it on a kitchen scale; density is a clue no photo carries, and a lab will want the weight.

Do not cut, grind or break the rock to look inside; photograph any existing broken or cut face in daylight at an angle. Most stony meteorites show scattered bright silver flecks of iron-nickel metal, like tiny bits of foil, through a gray or tan groundmass. Many also show chondrules: round grains about a millimeter across, packed like grit in concrete. Earth rocks do not contain chondrules, and pyrite, with its brassy color and cube shapes, is the usual false metal.

An iron meteorite shows uniform silvery metal on a cut face. Industrial iron does too, but it tends to show casting lines, a flat machined face, rust scale or a hollow interior.

Hold a strong magnet near the rock. Nearly all meteorites pull on it because of their iron-nickel metal. The pull is useful but not decisive: magnetite, some hematite, many basalts and most slag are magnetic too, and a few rare meteorite types are not. A rock that ignores a magnet is very unlikely to be a meteorite; a rock that grabs one still has to pass the other clues.

Rub a hidden corner of the rock once across the unglazed back of a ceramic tile. Hematite leaves a red-brown streak and magnetite a black one; a stony meteorite leaves a faint gray mark or none at all. Done on the underside it leaves no visible damage; keep it to one stroke.

A rock has to fit the meteorite column in most rows, not just one.

Two licensed meteorite photos show fusion crust and thumbprint-like dents. The scoria reference shows abundant gas holes to compare with those surface features.

Handle it as little as possible and do not clean, cut, polish, coat or glue it; labs want the original surface, and oils, lacquer and acids destroy evidence. Record the GPS location where it was found, the date and the weight. Keep it dry in a sealed bag.

Contact a nearby university geology department or meteorite laboratory and send the photos, weight and location first. Most will look at photos first and say whether to send a small sample. A classification takes months and involves a cut, a polished section and chemical analysis; nothing short of that confirms a meteorite.

Moldavite and other tektites are a different question. They are natural glass from impact melt, green or black, with pitted, grooved surfaces and no crust or metal; they are not meteorites, and a glassy green pebble found outside the known Central European strewn field is almost always bottle glass.