Desert varnish
Desert varnish is a thin, dark, shiny coating that builds up on rocks in deserts and can make Earth rocks look like meteorites.
In dry places, many rocks gradually develop a dark brown to black coating, often slightly glossy. This is desert varnish, also called rock varnish. It is extremely thin, usually far less than a millimetre, and is made mainly of clay minerals with manganese and iron oxides. It builds up very slowly, over thousands of years, and scientists still debate how much tiny microbes help to form it.
Desert varnish matters to meteorite hunters because deserts are among the best places to find meteorites. A dark, varnished pebble lying on pale desert ground can look a lot like a meteorite with fusion crust. Meteorites themselves can also become varnished and wind-polished after long periods lying in the desert, which is common on finds from Northwest Africa and Oman.
How do you tell the difference? Varnish usually coats only the side exposed to air and is patchy, while fusion crust covers the whole meteorite unless it has broken. Under a hand lens, fusion crust can show flow lines or a fine network of cracks. A chip or window through varnish often reveals a light, ordinary rock underneath. Remember that many deserts are protected areas, and collecting is often restricted by law.
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Level: Explorer Ages 10 and up