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.

Minerals can be opaque, meaning they block all light from passing through them; translucent, meaning they block some of the light; or transparent, meaning they pass most or all the light. A typical garnet or amethyst crystal is translucent; if you hold it up to a bright light, only a small fraction of the light entering the crystal ever reaches your eye, and you cannot see clear images through the crystal. Mica, a silicate mineral, can be cut into thin sheets so transparent that they serve as panes of a window. One colorless, transparent variety of calcite, dubbed "Iceland spar," exhibits a phenomenon called double refraction, which makes one object look like two. Another mineral, a borate called ulexite, occurs in thin parallel fibers that conduct light through them by total internal reflection, just like manufactured optical fibers. Ulexite seems to project an image onto the polished surface of the mineral, giving it the nickname "television stone."