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.

About 200 years ago, scientists studying electricity realized that metals allowed electrical currents to flow through them much more easily than most nonmetals. Because of its relative abundance, high electrical conductivity, and malleability (or ability to be shaped), copper has become the standard material for electrical wiring and switching. Certain types of low-iron silicates, such as muscovite and pyrophyllite, are naturally occurring electrical insulators. Glass or porcelain insulators, made from such minerals as quartz and feldspar, keep high-voltage power-transmission lines from touching their poles. Pyrolusite is the naturally occurring mineral version of manganese oxide, which is inside every non-rechargeable alkaline battery. Gasoline-powered articles start their engines with a jolt of electricity from lead-acid batteries, which get their lead from the mineral galena. Other rechargeable batteries contain nickel, cadmium, or other metals, found in a variety of ores.