Fulgurites are a form of natural glass that occurs when lightening strikes ground that has the right mix of ingredients (mostly sand) to form glass. With temperatures as high as 50,000 degrees farenheit (28,000 Celsius) -- or more -- lightening bolts product heat well above your typical glass furnace :-)
An interesting characteristic of fulgurites is the result of the rapid heating -- fulgurites are often tubular. That's because the sand melts and expands so quickly that a void is left in the center.
The longest known fulgurite is 17 feet (5.2 meters).
Fulgurites are sometimes referred to as petrified lightening.