When molten rock (magma) intrudes into the Earth's crust, it may heat up the rocks and minerals that it touches. The temperature may get as high as 700-800 degrees C (1,300-1,450 degrees F), which is still too low to melt the rocks near these igneous intrusions, but is hot enough to change the crystal structure of the minerals that make up these rocks. The resulting metamorphic rocks are known as hornfels. In some places, the heat stimulates the movement of water through tiny openings in the crust, dissolving minerals in some places and redepositing them in other places called hydrothermal veins. For thousands of years, humans have mined these veins for ores of important metals, such as iron, tungsten, tin, uranium, gold, and silver.

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.

Gemstones are minerals that are cut and polished to make beautiful "stones" for jewelry and other adornments. Humans traditionally consider diamonds, rubies, sapphires, and emeralds to be the four "precious" gemstones, with all other gemstones called "semiprecious." Many gems are transparent, meaning they allow some or most of the incident light to pass through them. To make a stone sparkle, gemcutters cut and polish the facets, or flat outer surfaces, of a gemstone to boost the number of times a light beam will reflect internally before leaving the stone and reaching the eye. Some semiprecious stones, such as malachite and lapis lazuli (a rock containing the mineral lazurite), are opaque to light; gemcutters polish them to a high gloss, without facets. This shiny, rounded type of gemstone is called a cabochon. Corundum (ruby or sapphire) specimens that contain tiny fibers of other minerals also may be cut into cabochons to create a luminous "star" effect on the stone's surface.