  Our ability to study the properties of the interstellar medium (ISM) in the
earliest galaxies will rely on emission line diagnostics at rest-frame
ultraviolet (UV) wavelengths. In this work, we identify metallicity-sensitive
diagnostics using UV emission lines. We compare UV-derived metallicities with
standard, well-established optical metallicities using a sample of galaxies
with rest-frame UV and optical spectroscopy. We find that the He2-O3C3
diagnostic (He II 1640 / C III 1906,1909 vs. O III 1666 / C III 1906,1909) is a
reliable metallicity tracer, particularly at low metallicity (12+log(O/H) < 8),
where stellar contributions are minimal. We find that the Si3-O3C3 diagnostic
(Si III 1883 / C III 1906,1909 vs. O III 1666 / C III 1906,1909) is a reliable
metallicity tracer, though with large scatter (0.2-0.3 dex), which we suggest
is driven by variations in gas-phase abundances. We find that the C4-O3C3
diagnostic (C IV 1548,1550 / O III 1666 vs. O III 1666 / C III 1906,1909)
correlates poorly with optically-derived metallicities. We discuss possible
explanations for these discrepant metallicity determinations, including the
hardness of the ionizing spectrum, contribution from stellar wind emission, and
non-solar-scaled gas-phase abundances. Finally, we provide two new UV oxygen
abundance diagnostics, calculated from polynomial fits to the model grid
surface in the He2-O3C3 and Si3-O3C3 diagrams.
