[en] A method for estimating the stellar metal abundances is proposed which compares measures of the equivalent width of a single feature in moderate resolution (1 A) optical spectra of stars, the Ca II K line at 3933 A, with models of the predicted line strength as a function of the broadband B-V color and Fe/H. The approach is capable of providing estimates of stellar metallicity over the range -4.5 to -1.0 with a scatter of about 0.15 dex for dwarfs and giants in the color range 0.33-0.85. For cooler stars, with B-V in the range 0.85-1.1, the scatter may be as large as 0.19 dex. The calibration of the Ca II K index with Fe/H is discussed, and average radial velocities and abundances are presented for several galactic globular clusters. 88 refs