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The thermal infrared spectral properties (7-12 mum) of natural rock surfaces from Silver Lake, CA, are investigated. Although the reflectance of weathered rocks is largely a function of the quartz content in rocks, the presence of desert varnish (clay coating) on rocks reduces the reflectance and spectral contrast with features unique to the rock spectra persisting if varnish is thin. Thick varnish has a spectrum with a reflectance peak near 9.6 mum, due to clays, and resembles the spectra of clay-rich playa surfaces. Comparison of laboratory reflectance spectra for varnish and weathered rock samples, with TIMS emissivity spectra for Silver Lake, suggest that TIMS signatures for felsic rocks are dominated by weathered rock and rock debris. In contrast, it is likely that varnish plays an important role in the TIMS signatures of mafic rocks.