Not all crystals form from dissolved chemicals. Some crystals start as liquids, such as magma (molten rock.) As the liquid cools and changes to a solid, it can form crystals. Just as wax freezes at a different temperature from water, different chemicals will crystalize at different temperatures, so some crystals form while other parts of the magma are still liquid. As the magma gets cooler and cooler, more and more minerals crystalize out. Want to see how a mixture of liquids can crystalize at different temperatures?