In pictures: Egypt's hidden treasures

Egypt is home to the only known field of natural silica glass.

The site lies in the Great Sand Sea, an expanse of more than a million square kilometres in Egypt's western desert, protruding into Libya.

Sand needs to be heated to 2000C for silica glass to form.

So how did this come about? One theory is that the sand was fused by the intense heat of a meteor impact.

But no crater has ever been found.