The forces of weather on the surface of the Earth can change the minerals that make up rocks. When mechanical weathering processes destroy old rocks, softer silicate minerals such as olivine and plagioclase may dissolve away, leaving behind harder minerals, such as quartz. In chemical weathering, the atoms react with oxygen or water in the environment. (Think of iron exposed for several months to the water and air outdoors. The iron rusts, and rusting is a kind of chemical weathering.) In the presence of water, some minerals transform into more hydrous minerals. Other minerals, such as calcite (calcium carbonate), dissolve completely in water over time. Some silicate minerals, such as quartz and garnet, are more resistant to weathering than other silicates, such as feldspar and mica.

Inside a mineral, atoms arrange themselves into a specific, repeating pattern called a crystal lattice or crystal structure. The smallest three-dimensional arrangement within the lattice is called a "unit cell," which is duplicated over and over again symmetrically. At the level of the everyday world, minerals that are growing without outside interference tend to form crystals that resemble their underlying crystal structures. Scientists call that kind of general, typical appearance a "crystal habit." Of course, conditions that existed during a mineral's formation or crystal growth may change its habit, but geologists still find this attribute to be a useful tool for identifying minerals. Scientists use more than three dozen adjectives to describe crystal habits. For example, natrolite and rutile can be acicular, or needlelike; quartz often forms hexagonal prisms; pyrite and halite typically crystallize as cubes; and mica is foliated or lamellar (layered).

Strictly speaking, coal is neither a rock nor a mineral; it contains the hardened, fossilized remnants of plants that lived millions of years ago. Coal deposits tend to form in sedimentary rock beds and metamorphose to "higher" grades under increasing pressure. First used as fuel by the early Chinese and Romans, the combustion of coal generates more modern-day electricity than any other fuel. Petroleum, the raw liquid that humans use for gasoline and oil, comes from the remains of dead organisms buried deep under sedimentary rock. The liquid pools in the cavities of porous rocks get mixed up with loose deposits of sands. Natural gas - naturally occurring gaseous carbon compounds, such as methane, given off by decaying organic material - also is found in or near coal and shale beds. Uranium, the fuel for generating electricity in nuclear power plants, occurs in uraninite and other ores that are mined around the world.