Fulgurite

Fulgurites are a rare type of natural glass made when lightning hits silica-rich minerals that are commonly in sand, soil, and some rocks. "Tubular fulgurites" are found in places with a lot of silica, like beaches or deserts. Lightning makes a tubular fulgurite when a bolt goes through the sand and melts silica into a liquid. The liquid silica cools and hardens quickly, leaving behind a thin glassy tube, usually with a rough outer surface and a smooth inner surface. Underground, a fulgurite may be shaped like the roots of a tree, branching out with many arms that trace the zigzagging path of the lightning bolt. They are quite delicate, with walls no thicker than 1-2 mm. Some of the largest fulgurites are removed from the ground in many pieces and then glued back into their original shape.