What happens when sand is struck by lightning?

If you're ever at the beach or out on a sandy dune and you see something that looks vaguely like a tree root made of glass, handle with care.

It's almost literally lightning come to Earth.

Well - perhaps not. But fulgurites, as these rare crystals are known, are the result of lightning strikes, with the tremendous energy petrifying the soil as it dissipates.

There are conditions. The lightning has to be at least 1800C or more in temperature - not a difficult ask considering a typical bolt can heat the air around it to be five times hotter than the sun's surface - and it won't work on just any soil.

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Fulgurites are most commonly found in very sandy earth. The lightning strike melts the sand particles, which as they cool, meld together and cohere with other particles around them.

They often resemble tree roots, mirroring the path taken by the lightning's dissipating energy.

Fulgurites form below the surface, but can be revealed when winds or tides brush away the surface sand.

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They are fragile and rarely can endure much exposure, especially without breaking apart.

A much-seen viral image purported to be of a fulgurite shows an arc of sand curving above a beach - but it's actually the work of a sculptor who goes by Sandcastle Matt on Flickr.

Typically, fulgurites measure about five to seven centimetres in diameter, but their length can vary. Most samples found have already been reduced to just several inches in length, but some can measure metres.

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And while most often seen emerging from the earth's surface, the "roots" can burrow extremely deep underground, depending on how far the lightning's energy travelled at hot enough temperatures.

Fulgurites are not considered to be precious per se, but they can be scientifically valuable.

Some samples discovered in the Sahara Desert, for example, dated back 250 million years and add to the theory that the region was once highly fertile and received frequent storms.

Fulgurites can also be found in placed where lightning has struck rock.

There, it will take the form of a coating of glass on the surface, or else as a filigree of glass that fills existing cracks within the rock.