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.

One clue to a mineral's identity is the way a sample breaks off a larger piece of mineral. If the mineral breaks to form fairly flat and smooth surfaces - planes of weakness in the crystal structure - geologists call it "cleavage." Mica and graphite have excellent cleavage because their atoms have strong bonds with each other within crystal planes, but only weak bonds between the planes. In other minerals, the atomic bonds have approximately the same strength in all directions, so they do not break into flat pieces when struck by a hammer. Scientists call this "fracture," and they use several adjectives to describe it. A "conchoidal" fracture has smooth, curved surfaces, while a "hackly" fracture is jagged, with sharp edges. Fracture may also be fibrous, splintery, or irregular.

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.