Our theoretical model, described in Sec. II, consists of a diamond coated with a cover layer, which we also sometimes refer to as a "protective layer". We use the fluctuation-dissipation theorem to obtain noise spectra at room temperature, and we calculate the effective capacitance and loss tangent. The surface cover materials we consider are: glycerol, propylene carbonate (PC), polymethyl methacrylate (PMMA), polyvinylindene flouride (PVDF), perflouropolyether (PFPE), and dimethyl sulfoxide (DMSO). PMMA, PVDF, PFPE, glycerol, and PC have been commonly used in experimental work [48][49][50][51][52]. PMMA and PVDF are both solids, while all the others are liquid at room temperature. We analyze the noise spectra at frequencies ranging from 1 kHz − 10 MHz, typical in experiments, and compare the results with experimental **data** of Ref. [32].