Can slag be mistaken for a meteorite?
Yes, very often. Slag, the glassy waste from smelting metal, can be dark, heavy, magnetic and rusty, but it is usually full of gas bubbles, glassy in places, and lacks the fusion crust and inner structure of a real meteorite.
Slag is probably the world's most common meteorwrong. When metal ores are smelted in a furnace, the unwanted rock and impurities melt together and are drawn off as a glassy liquid that hardens into slag. People have smelted iron for thousands of years, from small bloomery furnaces to the vast ironworks of the Industrial Revolution, so lumps of slag are scattered across fields, riverbanks, railway lines and old industrial sites, especially in countries such as Britain.
The confusion is easy to understand. Slag is often dark brown or black, can feel heavy, and may contain blobs of iron that make it attract a magnet and rust. A weathered lump can look rather like a meteorite with a rusty crust.
But slag usually gives itself away. Look for vesicles, the round holes left by gas bubbles, which are common in slag and almost absent in meteorites. Look for a glassy, shiny lustre on broken surfaces, or for swirls and drips that show it once flowed like treacle. Slag may contain bits of charcoal, brick or unmelted stone. Meteorites, by contrast, have a thin, even fusion crust, are solid inside, and in most cases show grey interiors with tiny flecks of metal and small round grains called chondrules.
Where a stone was found is a clue too. Something picked up near an old forge, mine or railway is far more likely to be slag than something from space. If you are still unsure, a museum or university geology department can tell the difference quickly, often from a photograph. Avoid testing a suspected meteorite with acid or heat, which is dangerous and can ruin a specimen.