Shale is the most abundant sedimentary rock and not particularly exciting to look at, but its looks belie its importance.

Shale is a fine-grained, fissile sedimentary rock that forms from the compaction of mud. It consists of microscopic and submicroscopic particles of silt and clay-size mineral particles.

The mineral grains are various combinations of quartz and three clay minerals — illite, kaolinite and smectite — with smaller amounts of carbonate minerals such as calcite and aragonite. The microscopic fissile layers that break apart readily are caused by parallel alignment of clay mineral flakes.

Shale comes in red, brown, yellow, green and black varieties depending on the environment in which the deposition occurred.

Black shales were deposited in anoxic, reducing environments and get their color from unoxidized carbon.

Black shales sometimes contain small amounts of heavy metals such as uranium, vanadium, molybdenum and zinc.

Large formations of shale that underlie many areas of all continents form by compaction and low-temperature baking due to burial at depths of up to 6 miles in Earth’s crust.

The fine particles of sediment that will become shale remain suspended in water long after larger particles of sand and silt have settled. They typically form far offshore or less commonly in lakes, lagoons and flood plains.

The composition of shales varies widely. Shales with large amounts of quartz grade into chert; those rich in carbonates grade into limestone; and those with excessive amounts of unoxidized carbon grade into coal.

Some shale contains enough organic material that it will burn and is used as a low-grade heat source in commercial applications.

Oil shale is rich in kerogens from which liquid hydrocarbons can be extracted. Estimates of global deposits of this shale oil are in the range of 3 trillion barrels, which is 2.5 times the estimated global reserves of petroleum.

Extracting the oil from oil shale is costlier than the production of conventional crude oil both financially and environmentally. Shales are also important in traditional petroleum deposits because the petroleum often resides in permeable sandstone beds between layers of impermeable shale. Petroleum migrates upward through the sandstone until shale at the crest of an anticlinal fold acts as a capstone to trap it.

Shale is porous and can hold a significant amount of liquid or gas, but its low permeability does not allow movement of these fluids through it.

Fracking, an increasingly controversial method of fracturing shale to release natural gas trapped within its layers, is widespread in the Marcellus formation that underlies much of the eastern United States. Shale plays a similar capstone role in directing ground water along aquifers where it might emerge at springs on hillsides where erosion has truncated the shale layer.

Shale might be the ugly stepsister to Earth’s gemstones, but it is essential to society nonetheless.