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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.

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