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.

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).

Shiny metals have been associated with beauty and wealth throughout human history. Gold, silver, and copper are three of the few metals that sometimes occur uncombined with other elements in nature, so it was relatively easy for ancient humans to collect them for personal adornment and trading. Copper jewelry emerged about 7,000 years ago, and the Egyptians began to make gold jewelry between 3,000 and 5,000 years ago. In many cultures, people put much of their wealth into their jewelry, both metallic and non-metallic (stones and bones), and used it as currency before the idea of money developed. The first coins appeared in the areas now known as Turkey and Greece about 2,600 years ago. Over the centuries, humans added so-called "base" metals to the gold and silver in coins, whose value became determined by law, not the actual value of the metals in the coin. Today, a U.S. "copper" penny is 97.5 percent zinc with a thin coating of copper; higher-denomination coins have a copper core covered with a copper-nickel alloy.