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 of the most striking, yet least diagnostic, features of many minerals is their color. Well-formed mineral crystals span the entire rainbow of tinctures, from red (cinnabar, garnet) to yellow (sulfur), green (malachite), blue (azurite, lazurite), and violet (the amethyst variety of quartz). Minerals containing iron and magnesium are often dark brown or dark green. Impurities, trace amounts of elements that do not normally belong in the mineral, may change the overall color of a crystal. For instance, depending on the trace amounts of impurities it contains, quartz may look colorless (no impurities), light pink (titanium, iron, or manganese), milky white (tiny bubbles of gas or liquid), purple (iron), yellow (iron), or brown (extra silicon). However, multiple minerals may have almost the same color, so scientists must rely on other physical properties to make definite identifications of mineral specimens.

Have you eaten any minerals lately? Almost certainly! Halite is the formal name of the mineral we use as table salt. Humans use salt not just to season food, but also to cure (preserve) meats. Clay minerals, part of the silicate group, serve as mild abrasives in toothpaste, while the fluoride comes from the mineral fluorite. Finely ground silicon dioxide is an anti-caking agent in many powdered foods such as gravy mixes and non-dairy coffee "creamer." The mineral trona is the primary source of sodium carbonate, which helps baked goods rise and gives toothpaste that foamy feeling in your mouth. Calcium sulfate from the mineral gypsum coagulates (or solidifies) tofu, a soybean-based food. Many of the plants we eat depend on mineral-based fertilizers for robust growth.