Jet

Jet is a geological material that is not considered a mineral in the true sense of the word. It is a hard, compact variety of lignite, coal-black in color. It is easily polished and has been used in the manufacture of cheap ornaments.

A jet is a stream of fluid produced by discharge through an orifice into free space.

Since gas is a compressible fluid, the velocity attained by a jet of gas cannot be determined by Bernoulli's principle. Instead, if one uses adiabatic expansion from P₁ to P₂ in a properly shaped nozzle, the ideal velocity of a gas jet is:

v = 8×√((RT₁/z)×(1-(P₂/P₁)^z))

where R and z are characteristics of the gas, R being the common gas constant and z being (c_p-c_v)/c_p, where the c's are the specific heat at constant pressure and constant volume, respectively. z = 0.286 for air under 500 °C. z = 0.23 to 0.28 for most combustion products of fuels. T₁ is the absolute temperature at pressure P₁.

The term jet is also an abbreviation for jet engine or jet aircraft.

Jet is commonly used as a description, usually referring to the soot that accumulates at the back of a jet engine. EG: "Jet Black".