A fulgurite is formed when lightning strikes dry quartz sand. The temperature from electrical resistance in the ground can get so high that the sand melts (about 1800°C), and later hardens into fragile tubes of lechatelierite [silica glass]. These tubes are the fulgurites. They can also be easily produced in a laboratory using an electric furnace.
The Latin word for lightning, fulgur, is the root of the word fulgurite, meaning fused sand.
When excavating fulgurites, scientists use techniques developed by palæontologists for removing dinosaur fossils - appropriate for something that might be considered fossilised lightning.
The longest known fulgurite was found in Florida. It consisted of three branches totalling 11.6 metres. Fulgurites are inevitably excavated in pieces, as the glass is so fragile.
The colour of the fulgurite depends on the composition of the sand they are formed in. Black and tan are most common, although an almost-translucent white example was found in Pensacola, Florida. The interior is normally very smooth or lined with fine bubbles, which are formed when moisture is trapped in the glass as the fulgurite cools suddenly after creation. The exterior is coated with rough sand that adheres to the glass as it cools.
Unfortunately, I forgot to keep a note of where this example was collected. >_<