See, lightning strikes sand at the right angle - it crystallizes into the perfect shape of itself.
Fulgurites are natural hollow glass tubes formed when quartzose sand, silica, or soil is struck by lightning. They appear to be used primarily for powerful summoning spells.
Bobby and the Winchesters obtain a binding spell from Crowley designed to enslave Death. One of the ingredients for the spell is "an act of God crystallized forever" which Bobby deduces is fulgurite, which is formed when lightning (an act of God) fuses the silica in sand. Sam, Dean, and Bobby drive to the home of Dr. and Mrs. Weiss, a wealthy couple who recently bought a large piece of fulgurite at an auction. They get the fulgurite and mash it up, allowing them to summon and bind Death.
After extensive research in the Bunker, Dean finds a journal from a Greek hunter named Drakopoulos that was recovered and translated by the Men of Letters. It detailed a method of summoning, trapping, and potentially killing Zeus. The summoning spell involves a bone from one of his his worshippers, frozen energy from the hand of Zeus, which Dean assumes means fulgurite. The journal states that Zeus can be killed by a wooden stake made from a tree struck by lightning. Hayley tells Dean that fulgurite is found in many New Age shops where it is used in making cheap jewelry. Dean and Hayley buy the fulgurite and later cast the spell, summoning and trapping Zeus.
After losing any hope of conquering the effects of the Mark of Cain, Dean decides to summon Death to either kill him or get rid of it. Somehow, Dean is able to collect more fulgurite to summon and bind Death to him again.
Fulgurite in Lore
In some New Age spiritual practices, fulgurite is said to enhance the power of prayer, and connections to angels (Source).