The Earth’s crust is dominated by oxygen, silica, and aluminum although about 100 elements exist—about 20 non-metals and 80 metals. They exist in minerals, which have a somewhat definite chemical composition and structure, and rocks, which are consolidated aggregates of one or more minerals. Minerals exist in fixed amounts; and some minerals may be too rare or unavailable to satisfy all demands. Pure metals are generally lustrous; malleable; flexible; strong in compression, tension, and torque; and hard. Metals can be subdivided into groups with more common properties. Non-metal elements can form long covalent molecules or radicals that bond with metals. Minerals are used in a variety of ways, quantities, and chemical forms. Forms include amorphous, covalent molecule, pure crystal, ionic compound. Mineral elements, molecules, and compounds are created, concentrated, and/or found in igneous, metamorphic, and sedimentary geologic formations. They are mined in various methods including shaft, open pit, hydraulic, placer, fracking, and acid leach. These have various degrees of difficulty, danger, and impact on the environment.