Quartz, silicon oxide (silica), is one of the most abundant and ubiquitous minerals in the earth's crust. It is the primary constituent of beach sand and occurs in many colors, all of which are used as gemstones. Microcrystalline (cryptocrystalline) varieties can be stained by mineral impurities to produce a wide range of colors and patterns. Chalcedony is a translucent near-colorless material; if the impurity staining is in layers or bands, the resulting material is generally called agate. If the coloration is irregular, or in odd shapes or random patterns, the term jasper is usually applied. Hundreds of localities for these materials exist worldwide, and the varieties in color and pattern are almost endless. The photo shows a cross section of a unique material formed by the replacement by silica of a dense aggregate of fragmented clamshells that had settled to the floor of a marine environment.