Constituents of sand
The most common constituent of sand, in inland continental settings and non-tropical coastal settings, is silica (silicon dioxide, or SiO2), typically in the form of quartz, which, because of its chemical inertness and considerable hardness, is resistant to weathering. The composition of sand varies according to local rock sources and conditions. The bright white sands originate in tropical and subtropical coastal settings are ground-up limestone. Arkose is a sand or sandstone with significant feldspar content which is derived from the weathering and erosion of a (usually nearby) granite. Some locations have sands that contain magnetite, chlorite, glauconite or gypsum. Sands rich in magnetite are dark to black in color, as are sands derivative from volcanic basalts. The chlorite-glauconite bearing sands are usually green in color, as are sands derived from basalts (lavas) with a high olivine content. The gypsum sand dunes of the White Sands National Monument in New Mexico are famous for their bright, white color. Sand deposits in some areas have garnets and other resistant minerals, with some small gemstones.