What is fusion crust and how do I recognise it?
Fusion crust is the thin glassy skin that forms when a meteorite's surface melts during its fiery fall. On a fresh stony meteorite it is usually black, matt and less than about a millimetre thick, over a paler interior.
A meteoroid enters Earth's atmosphere at many kilometres per second. Air in front of it is squeezed and heated, and the rock's outer surface melts and streams away, a process called ablation. Once the meteoroid slows down high in the atmosphere, melting stops and the last film of molten rock solidifies within seconds. That frozen film is the fusion crust. Because the heating lasted only moments, the interior usually stays cool and unaltered.
On most stony meteorites, fresh fusion crust is black or very dark brown and dull or satiny rather than glossy, often with fine contraction cracks forming a network of polygons. It is thin, typically less than a millimetre. If a corner has been chipped, you will see the sharp contrast between that dark skin and the lighter grey interior, a classic sign of a meteorite. Some achondrites behave differently: eucrites from the asteroid Vesta often have a shiny, glassy black crust, because their melted surface forms more glass.
Look closely and the crust can record the fall. On oriented meteorites, which kept the same face forward as they flew, molten rock flowed back from the front and froze into fine flow lines, sometimes with a lip of crust around the back edge. Areas of thinner crust may mark where a stone broke apart late in its flight.
Crust does not last forever. Within months to years on the ground it begins to weather to brown, and in time it may flake away. Many Earth rocks also have dark coatings, such as desert varnish, so compare the colour, thickness and the inside before deciding. Keep a suspected meteorite dry, and never scrub, heat or apply acid to the crust.