Fusion crust
Fusion crust is the thin, dark skin that forms on a meteorite as its surface melts during its fall.
As a meteorite races through the air, its outside gets so hot that it melts. When it slows down, that melted layer cools very quickly and hardens into a thin coat called fusion crust. It is usually less than a millimetre thick, like the shell on a chocolate-covered sweet, while the inside of the rock stays unchanged.
Fusion crust is one of the best clues that a rock came from space. On most stony meteorites a fresh crust is black and dull, a bit like a matt paint. Some meteorites, such as the eucrite that fell at Stannern in what is now the Czech Republic in 1808, have a crust that is shiny and glassy. Over years on the ground the crust slowly rusts and turns brown.
To recognise fusion crust, look for a thin dark coat that covers the rock, with a lighter inside showing where a corner has chipped. Many Earth rocks have dark coatings too, so let an expert check. Never scratch, cut or heat a possible meteorite yourself.
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Level: Beginner Ages 7 and up