Shale is a very common form of sedimentary rock found in deposits all over the world. It is distinguished by being soft and highly fissile. Shale has a number of ornamental and practical uses, in addition to being a rich source of fossil depositions which can provide information about different eras in Earth's geological history.
This sedimentary rock starts as a deposition of sand, mud, and clay, with very fine particles of other minerals such as quartz. Over time, the deposition slowly compresses, eventually turning into shale. One of the defining characteristics of this type of rock is the layers of material which can be seen when the rock is cut crosswise, showing the subsequent layers of deposition which occurred, with the rock tending to sheer or fracture along these deposits. The very fine grain of the particles is another distinguishing characteristic.
Shales come in an array of colors. Some are so dark that they are almost black, while others may be greenish, bluish, brown, or cream, depending on the precise composition of the rock. In the case of shale with fossil deposits, the deposits form as a result of biological material which became trapped in the shale while it was being deposited and subjected to pressure. In some cases, parts of the organism may be represented in mineralized form, while in others, the rock holds only the impression of a organism which decayed after the rock started to harden.
In manufacturing, shale is used as a filler material for concrete and brick. People sometimes confuse this rock with slate, a much harder rock which can be used for a wide variety of purposes including roofing, tiling, blackboards, and so forth. In the case of slate, the rock is subjected to metamorphic processes which change the structure and properties of the rock, making it harder and more durable. Although slate also bears distinctive layers and tendency to split along these layers when stressed, it is much harder than shale.
A type of rock known as “oil shale” is of interest to some people who work in the energy field. Oil shale is not necessarily a shale, although it sometimes is, and it shares many of the traits associated with true shales. It bears a mixture of chemical compounds which can be extracted and used to generate energy. However, tremendous energy is needed to extract these compounds, making this rock appealing only in regions where other sources of oil for energy have been exhausted.