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We demonstrate diamond growth through microwave plasma-enhanced chemical vapor deposition using a sealed (static-mode) CH4/H2 process gas mixture. The growth experiments were complemented by spatially and spectrally resolved optical emission imaging measurements of electronically excited C2 and CH radicals in the hot plasma core. The as-grown material was characterized by Raman Spectroscopy and Scanning Electron Microscopy and shown to be typical of polycrystalline diamond grown using traditional methods. Moreover, this material was essentially indistinguishable from material grown using a tracked flow-mode of operation in which the input methane flow rate was progressively reduced to mimic the time evolving C2 emission intensities in the static-mode experiments. These proving static-mode studies demonstrate a ~30-fold improvement (cf. that achieved using standard flow-mode conditions) in the conversion efficiency of carbon in the input source gas into diamond, and we argue that further gains should be possible with appropriate reactor and process optimization. Static-flow growth could be particularly advantageous in the case of depositions using limited, expensive, hazardous, or environmentally damaging feedstock gases.