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.