Crystals are solid materials whose atoms are arranged in a highly ordered, repeating pattern. This unique structure gives crystals their distinct shapes and properties, such as clarity and color. Common examples of crystals include quartz, amethyst, and salt. They can form naturally in the environment or be created artificially in laboratories. Crystals are often categorized based on their geometric shapes and the arrangement of their atoms. There are several types of crystal systems, including cubic, hexagonal, and tetragonal. These systems help scientists understand the physical and chemical properties of different crystals, which can be used in various applications, from electronics to jewelry.