Crystals are made from fluids by Crystallization, which is a kind of phase change. Usually the fluid is a liquid, but crystals can form from a gas too. For example, snow is water crystals (ice) formed from air that contains water vapor (evaporated water). Crystallization can be by freezing from a simple fluid or from a solution. Sometimes heat and pressure needs to be applied to a substance for crystals to form. Diamond is a carbon crystal formed under heat and pressure. Crystals are found naturally or can be made artificially.