Quartz, silicon oxide, 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. Other varieties of quartz used as gems include those with other minerals trapped inside. The best known is rutilated quartz, containing needles and sprays of the mineral rutile (titanium oxide), available chiefly from Brazil.