Clastic rock The term "mud" is used when clay and silt particles are mixed in the sediment; "mudrock" is the name of the rock created with these sediments. Furthermore, particles that reach diameters between .062 and 2 millimeters fall into the category of sand. When sand is cemented together and lithified it becomes known as sandstone. Any particle that is larger than two millimeters is considered gravel. This category includes pebbles, cobbles and boulders. Like sandstone, when gravels are lithified they are considered conglomerates. Conglomerates are coarse grained rocks dominantly composed of gravel sized particles that are typically held together by a
Sand whole numbers. The most common constituent of sand, in inland continental settings and non-tropical coastal settings, is silica (silicon dioxide, or SiO), usually in the form of quartz, which, because of its chemical inertness and considerable hardness, is the most common mineral resistant to weathering. The composition of mineral sand is highly variable, depending on the local rock sources and conditions. The bright white sands found in tropical and subtropical coastal settings are eroded limestone and may contain coral and shell fragments in addition to other organic or organically derived fragmental material, suggesting sand formation depends on living organisms, too.