Silicon and oxygen are two of the most common elements on Earth. Together, they make up nearly 75 percent of the Earth's crust, so it is no surprise that they play major roles in forming many of the minerals that we see in rocks. The silicon atom, which has four electrons in its outermost region, likes to form chemical bonds with oxygen atoms, which are attracted to extra electrons. Thus, silicon and oxygen, together with some of the metallic elements, can combine to make hundreds of different minerals. For example, quartz has two oxygen atoms for every silicon atom, and feldspar has two or three silicon atoms grouped with eight oxygen atoms and a few metal atoms. About half of the most common minerals found on Earth belong to the silicate group, as do some beautiful gemstones such as amethyst, opal, and topaz.

One clue to a mineral's identity is the way a sample breaks off a larger piece of mineral. If the mineral breaks to form fairly flat and smooth surfaces - planes of weakness in the crystal structure - geologists call it "cleavage." Mica and graphite have excellent cleavage because their atoms have strong bonds with each other within crystal planes, but only weak bonds between the planes. In other minerals, the atomic bonds have approximately the same strength in all directions, so they do not break into flat pieces when struck by a hammer. Scientists call this "fracture," and they use several adjectives to describe it. A "conchoidal" fracture has smooth, curved surfaces, while a "hackly" fracture is jagged, with sharp edges. Fracture may also be fibrous, splintery, or irregular.

When some minerals are ground into powders, they may be used as pigments, solid substances that change the color of the materials they are mixed with. The first pigments, known since prehistoric times, were iron oxides, which make warm, dark reds and browns. Other early pigments were made of lead, carbon (in charcoal form), malachite, and azurite. Today's paints may contain synthetic dyes in addition to natural pigments and fillers such as calcium carbonate, mica, silica, talc, and titanium dioxide. Minerals have been used as ingredients in facial makeup at least since the days of ancient Egypt, when women painted their eyes with kohl, a mixture containing finely ground galena, or lead sulfide. Modern-day cosmetics manufacturers avoid hazardous substances like lead and arsenic, but many of their products contain microscopic particles of minerals and their derivatives such as iron oxide (from goethite and other minerals), titanium dioxide (usually from ilmenite), talc, mica, and kaolinite.