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.

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."