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.

Strictly speaking, coal is neither a rock nor a mineral; it contains the hardened, fossilized remnants of plants that lived millions of years ago. Coal deposits tend to form in sedimentary rock beds and metamorphose to "higher" grades under increasing pressure. First used as fuel by the early Chinese and Romans, the combustion of coal generates more modern-day electricity than any other fuel. Petroleum, the raw liquid that humans use for gasoline and oil, comes from the remains of dead organisms buried deep under sedimentary rock. The liquid pools in the cavities of porous rocks get mixed up with loose deposits of sands. Natural gas - naturally occurring gaseous carbon compounds, such as methane, given off by decaying organic material - also is found in or near coal and shale beds. Uranium, the fuel for generating electricity in nuclear power plants, occurs in uraninite and other ores that are mined around the world.