Why does a meteorite have a black crust?
The black skin is fusion crust: the meteorite's surface melted during its fiery plunge through the atmosphere and then froze into a thin glassy coating.
As a meteoroid tears through the atmosphere at many kilometres per second, the air in front of it is compressed and heated to thousands of degrees. The rock's outer surface melts and is swept away in a process called ablation, which is what makes the bright trail of a meteor. When the meteoroid has slowed enough that the heating stops, the last film of molten rock left clinging to its surface cools almost instantly, freezing into a thin glassy shell. This is the fusion crust.
Fusion crust is usually less than a millimetre thick. On stony meteorites it is typically dull to matt black, sometimes with fine cracks or ridges, while on some calcium-rich achondrites, such as eucrites from the asteroid Vesta, it can be glossy and shiny. The crust's colour comes from the iron in the original rock: melting and quick cooling turn iron-bearing minerals into a dark glass with tiny crystals of magnetite.
Look closely and the crust can tell a story. Flow lines, tiny streaks of melt frozen in place, show which way the stone was pointing. A stone that held a steady orientation may have a smooth front and a frothy, thicker crust at the back. A patchy, thinner secondary crust shows the stone broke apart in flight and the new faces were only briefly heated.
Fresh crust does not last forever. Over years to centuries, rain and air turn it brown as iron rusts, and in deserts it can be sandblasted away. A crisp black crust is therefore a sign of a recent fall. It is also one of the best clues for spotting a meteorite, though shiny black slag and desert-varnished rocks can look similar, so an expert should always confirm.