Deep underground, magma (molten rock) heats water, which is less dense than cold water, so it tends to rise toward the surface. The hot water can carry dissolved minerals into fissures and cracks in the Earth's crust. When minerals are dissolved in a liquid, scientists say that the minerals are in solution. Eventually, the water cools and the materials in the water crystallize out of solution in their new location, and the resulting minerals look different from the surrounding rocks. Hydrothermal comes from the Greek words for water and heat; these structures are called hydrothermal veins because they resemble the blood vessels in the bodies of animals. Many ores of economically important metals, such as lead, zinc, copper, and gold, are found in these veins.

About 200 years ago, scientists studying electricity realized that metals allowed electrical currents to flow through them much more easily than most nonmetals. Because of its relative abundance, high electrical conductivity, and malleability (or ability to be shaped), copper has become the standard material for electrical wiring and switching. Certain types of low-iron silicates, such as muscovite and pyrophyllite, are naturally occurring electrical insulators. Glass or porcelain insulators, made from such minerals as quartz and feldspar, keep high-voltage power-transmission lines from touching their poles. Pyrolusite is the naturally occurring mineral version of manganese oxide, which is inside every non-rechargeable alkaline battery. Gasoline-powered articles start their engines with a jolt of electricity from lead-acid batteries, which get their lead from the mineral galena. Other rechargeable batteries contain nickel, cadmium, or other metals, found in a variety of ores.

Ores are rocks that contain minerals rich in elements that are valuable to human society. Almost all the metals we use - iron, aluminum, lead, copper, zinc, uranium, and others - come from ores. To form ores, elements and compounds must be concentrated via one of several processes. In hydrothermal processes, hot water seeping through the ground may concentrate metal-rich minerals into veins. Dense minerals from which we get metals such as platinum, nickel, and chromium crystallize and settle out of some types of magma (molten rock) underground. Much of the iron we use comes from banded iron formations: rock built up from layers of sediments containing iron oxides. Other minerals become concentrated through erosion, transport, and deposition of small rock grains. Mining began in ancient Egypt, Greece, and Rome, followed by a steady stream of technological improvements during the medieval and early modern eras. Today miners use specialized heavy equipment for both surface and underground mining.