Understanding the development history of our Earth and life on Earth. Geology is primarily a descriptive and explanatory science, but ultimately it is a historical science. Closely connected with geology as a core science are several auxiliary sciences, such as mineralogy, the study of minerals, meaning the individual building blocks of the solid Earth's crust (coals, ores, salts, gemstones, and components of rocks). Additionally, there is the study of rocks or petrography and petrology, which is the study of rocks (mineral masses composed of minerals that make up the Earth's crust). Furthermore, there is paleontology, the study of plant and animal life from earlier periods, and finally, the study of deposits, which examines the occurrence and causes of sediment formations.