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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 (cp-cv)/cp, 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".