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Biologic sedimentary rocks form from once-living organisms.They may form from accumulated carbon-rich plant material or from deposits of animal shells.Shale is a rock made mostly of clay, siltstone is made up of silt-sized grains, sandstone is made of sand-sized clasts, and conglomerate is made of pebbles surrounded by a matrix of sand or mud.Chemical sedimentary rocks are formed by chemical precipitation.[Sources: Mostly from USGS, Cliff Notes (Weathering and Soil) and newspaper articles] Geologist classify the Earth's rocks into three categories: 1) igneous, formed by magma within the Earth;.2) sedimentary, formed when sand, clay or other materials are deposited by water, wind or glacial action and cemented together by pressure and chemical reactions; and 3) metamorphic, sedimentary or igneous rocks that have been later modified by heat and pressure or natural processes. Most gems are crystals formed by the cooling of hot gases, solutions and melts deep inside the earth.They form from deposits that accumulate on the Earth's surface.
Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot, mineral-rich fluids or, more commonly, some combination of these factors.
This process begins when water traveling through rock dissolves some of the minerals, carrying them away from their source.
Eventually these minerals are redeposited when the water evaporates away or when the water becomes over- saturated.
Rapid cooling means the individual mineral grains have only a short time to grow, so their final size is very tiny, or fine-grained Sometimes the magma is quenched so rapidly that individual minerals have no time to grow.
This is how volcanic glass Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms.