Is the lump of iron in my garden a meteorite?
Probably not. Most iron lumps found in gardens are man-made scrap, slag or iron ore. Meteoritic iron contains nickel and shows a crystal pattern when etched.
Iron meteorites are rare, but man-made iron is everywhere, especially near old farms, industrial areas, railways and buildings. Gardens can contain buried scrap, pieces of old tools, nails, pipes, cast-iron fittings, slag from bonfires and forges, and even cannonballs.
Signs of man-made iron. - Regular shapes, flat faces, holes, threads or casting marks. - Bubbles or a glassy, swirled texture, typical of slag. - Location in soil with other debris, such as brick, pottery or glass. - A uniform grey interior when filed or cut, with no crystal pattern.
Signs that could suggest a meteorite. - An irregular, rounded shape with smooth thumbprint-like hollows, called regmaglypts. - A thin brown or bluish-black crust. - A heavy feel for its size. - No signs of manufacture.
Tests. The most decisive test is nickel content. Meteoritic iron contains about 5 to 30 per cent nickel, while most man-made iron contains little or none. Laboratories can test for nickel using XRF or chemical methods. An etched polished surface showing a Widmanstätten pattern or Neumann lines would strongly suggest a meteorite, but etching requires care and is best done by experts.
Next steps. Do not cut, heat or grind the lump. Photograph it with a scale, note where and at what depth it was found, and send pictures to a museum or university. Some museums offer identification days.
Even if it turns out to be historical iron, it may be of interest to local historians or archaeologists.