Geologists classify rocks based on their composition and how they formed. One of the three main categories is sedimentary rock, which includes all rocks that form through accumulation of sediment. Some so-called clastic sedimentary rocks are made when pieces of rock or debris build up over time. Chemical and organic sedimentary rocks, by contrast, form through a variety of processes.
Organic or biological sedimentary rocks are formed by living organisms, typically when the remains of living organisms build up and are compacted by sediment. Coal, for example, is made from long-dead vegetation crushed by thick layers of sediment and chemically altered through heat and pressure. Most limestone deposits are made from the shells of microscopic sea organisms. Coral reefs are a beautiful example of organic sedimentary rocks made by creatures that are still living -- corals that build their own homes from calcium carbonate.
Chemical sedimentary rocks, by contrast, form when conditions favor a chemical reaction or process that causes chemicals dissolved in water to precipitate, creating a layer of sediment. When water in a salty sea or lake evaporates, for example, it may leave behind salt and gypsum deposits. In calcium-rich waters, changes in temperature or acidity may cause calcium carbonate to precipitate. Accumulation of calcium carbonate deposits can lead to the formation of limestone. Sometimes magnesium in water that enters the pores of a limestone rock can replace the calcium in the rock, turning limestone into another chemical sedimentary rock called dolostone.
Both organic and chemical sedimentary rock form through accumulation of sediment. This makes them very different from igneous rocks, which form when lava or magma cools and solidifies, or metamorphic rocks, which form under high heat and pressure. Some sedimentary rocks may be either organic or chemical, depending on how they formed. Limestone, for example, may be created from either organic or chemical processes.
The key difference between organic and chemical sedimentary rocks is the process that forms them -- and often their texture, composition and appearance bear mute witness to that process. Geologists can determine whether a sedimentary rock is organic or chemical by looking at its texture. Organic sedimentary rocks contain fossilized remains of living creatures, since it is these remains that accumulated to form the rock in the first place. Chalk deposits, for example, often contain microscopic fossils. Salt deposits formed from evaporation, by contrast, usually contain a mixture of salts, just as you would expect in a rock that formed from the evaporation of a salty lake.