* Coal: Formed from the accumulation and compaction of plant matter, primarily in swampy environments. The high carbon content in coal comes directly from the organic remains.
* Oil Shale: Contains kerogen, a waxy organic matter derived from ancient algae and plankton. Oil shale is not oil itself, but it can be heated to extract oil.
* Diatomite: A porous rock made up of the fossilized remains of diatoms, single-celled algae with silica shells.
* Coquina: A limestone composed primarily of shell fragments, often from marine organisms.
* Chalk: A type of limestone formed from the accumulation of microscopic marine organisms called coccolithophores.
It's important to note: The term "organic" in geology refers to the origin of the rock from once-living organisms. It doesn't mean that the rocks are made of the same complex molecules as the living things they came from. Over time, the organic material undergoes significant transformation due to heat and pressure.
