Mineral crystals are solid substances formed by the orderly arrangement of atoms. They occur naturally in various shapes and sizes, depending on the type of mineral and the conditions under which they form. Common examples include quartz, feldspar, and calcite. These crystals can be found in rocks, soil, and even in some living organisms. The process of crystal formation is called crystallization, which can happen through cooling molten rock, evaporation of water, or changes in pressure and temperature. Each mineral has a unique crystal structure, which determines its physical properties, such as hardness and color. Crystals are often studied in geology and materials science for their beauty and practical applications.