Campo Del Cielo - Fer, IAB-MG - Coupe polie et gravée - 6.32g - Argentine

This 128.72g Mojave Desert fulgurite is a thick, gently curving tube of fused sand, its surface studded with pale and dark mineral grains caught in the melt. It is photographed beside its collection card and a 1cm scale cube.

The Mojave Desert of southern California has wide stretches of dry, sandy ground, and its summer thunderstorms bring lightning down onto it. When a strike enters loose sand, the heat fuses the grains around the channel into a rough tube or branching root, with a glassy, often hollow interior and an outer crust of partly melted sand grains still attached. These sand fulgurites are the classic form that earned fulgurites the popular name “petrified lightning”.

A fulgurite forms in a fraction of a second. The temperature inside a lightning channel can exceed 30,000 kelvin, and where the discharge enters the ground it melts the surrounding soil or rock, which then cools quickly to glass. Fulgurites are classed as mineraloids rather than minerals because they are largely non-crystalline. Researchers group them by the material they formed in, from tubes in sand to glassy crusts and droplets on soil and rock, so the shape and colour of each piece reflect the ground the lightning struck.

Fulgurites are not meteorites, but they sit naturally alongside tektites, impact glasses and other natural glasses as products of sudden, extreme heat. For collectors, Mojave fulgurites show the branching, root-like form of a lightning strike in sand, with a clear North American desert locality, and no two pieces are alike.