jet

Jet is a black, organic mineraloid formed from fossilized wood altered by heat and pressure during the Carboniferous period. It is not crystalline, giving it a mineraloid classification; it typically shows a glossy to submetallic luster and a relative softness (Mohs hardness about 2.5). It is brittle and fractures with smooth, planar cleavage.

Jet has long been used as a gemstone. It is cut into beads and cabochons and has been valued in jewelry since antiquity, with a notable historic source at Whitby, England. The material can be polished to a deep black finish and is sometimes stained or treated to alter color.

Jet propulsion uses engines that accelerate air and exhaust rearward to produce thrust. The common forms are turbojet and turbofan, which compress air and burn fuel, and ramjet or scramjet designs that rely on high flight speed for air compression. The first practical jet engines appeared in the 1930s, developed by Frank Whittle in Britain and Hans von Ohain in Germany, and were deployed during World War II. Today, turbofans power most commercial aircraft; military jets use turbojets or advanced turbofans; ramjets and scramjets are used for high-speed research and some missiles.