A crystal is a solidified form of a substance in which the atoms or molecules are arranged in a definite pattern that is repeated regularly in three dimensions. Crystals form when saturated or supersaturated solutions deposit layer upon layer or a substrate, initially onto a surface and then upon each other. This process generally forms a solid with remarkable geometric properties. Because of the mechanics of crystal formation, the slower the crystal forms, the larger and more perfect it will ultimately be.

The beauty and perfection of crystals has long made them prized possessions for human beings. Diamonds are generally thought to be the most valuable and beautiful crystals. They are composed of a lattice of pure carbon, as is graphite, buckminsterfullerum, and carbon nanotubes. The regular structure, and the properties of carbon, make all of these forms exceptionally strong.

Other fascinating properties of crystals are exploited in electronics: quartz crystals oscillate with sufficient regularity to be used as timepieces, semi-conducting crystals form the basis for modern electronics. The quantum properties of the crystal structure allow fascinating electric properties: one can actually make a transistor from a carbon nanotube twisted properly. Our increasing understanding of quantum mechanics and crystals has allowed the development of the laser and numerous other inventions. The method in which crystal structure is determined is a process called x-ray crystallography. It was through these means that the structure of DNA (deoxyribonucleic acid) was determined by Watson and Crick. According to the principle of evolutionary pressure and selfish gene theory, some people have offered the supposition that the development of life on earth, and perhaps elsewhere in the universe, began with crystals.