Shale

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The proportion of clay to other minerals varies. Typically dark gray to black, but can be red, brown, or green depending on impurities such as iron oxides or organic matter. Has a dull luster. It is characterized by its fissility, meaning it tends to split into thin layers (laminae) along bedding planes. Shale is relatively soft, with a Mohs hardness typically between 2.5 and 4. It does not exhibit distinct crystal structures visible to the naked eye. Cleavage is excellent parallel to bedding.

Shale forms from the compaction of mud (silt and clay-sized particles) in calm water environments such as lakes, rivers, deltas, and continental shelves. As layers of mud accumulate and are buried, water is expelled, and the sediments are lithified into rock through primary compaction and cementation. The fine grain size indicates a low-energy depositional environment where fine particles could settle out of suspension. Shales can range in geological age from the Precambrian to the Cenozoic, forming throughout Earth's history in various basins.

Shale is used in the production of ceramics, bricks, and Portland cement (when mixed with limestone and heated). Organic-rich shales, known as oil shales, can be a source of oil and natural gas if the kerogen within them is mature and extractable. Historically, certain shales were used as roof tiles. It is also sometimes used as a filler material in construction and road building.

Shale is the most abundant sedimentary rock, making up about 70% of the sedimentary rocks in the Earth's crust. It is often associated with fossil discoveries, as the fine-grained nature and rapid burial in low-oxygen environments preserve delicate organisms very well. The fissile nature of shale is due to the parallel alignment of clay minerals during compaction. Black shales are often rich in organic matter and can be significant source rocks for hydrocarbons.

Shale can be identified in the field by its fine grain size, the ability to split into thin, platy layers (fissility), and generally dull appearance. It often feels smooth or slightly gritty to the touch. The color can be an indicator of its composition (e.g., black for organic matter, red for iron oxides). It is commonly found in sedimentary basins worldwide, often interbedded with other sedimentary rocks like sandstones and limestones. Collectors might look for shale for its fossil content or to observe its characteristic fissility. When wet, shale can become slippery and soft, a useful field test. To confirm, one might attempt to split a piece with a rock hammer; true shale will readily separate into layers.