Answer: Lightning

With a little luck, a careful eye, and a receding storm on the horizon, a trip to the beach might just yield a really unique find: fulgurite.

Fulgurite is a mineral (or, more technically, a mineraloid as it lacks a uniform crystalline structure) that forms when a lightning strike hits sand. In that intense and brief moment where the sand is superheated to around 3,272 degrees Fahrenheit (1,800 degrees Celsius), a hollow, branching tube, crust, or vesicular mass of cloudy glass is formed around the path of the lightning.

Fulgurite belongs to the family of mineraloids know as lechatelierite, all formed by the introduction of high pressure and/or temperature to a body of silica. When lechatelierite is formed as the result of a meteorite impact, for example, the resulting blobs of glass (shaped that way because they cool as they fly away from the impact site) are known as tektites.