We report the observation and measurement of the rate of diffractive d
ijet production at the Fermilab Tevatron (p) over bar p collider at ro
ot s = 1.8 TeV. In events with two jets of E-T > 20 GeV, 1.8 < \eta\ <
3.5, and eta(1) eta(2) > 0, we find that the diffractive to nondiffra
ctive production ratio is R-JJ = [0.75 +/- 0.05(stat) +/- 0.09(syst)]%
. By comparing this result, in combination with our measured rate for
diffractive W boson production reported previously, with predictions b
ased on a hard partonic pomeron structure, we determine the pomeron gl
uon fraction to be f(g) = 0.7 +/- 0.2.