Microwave emissivities have been calculated over the globe from satellite observations between 19 and 85 GHz with the Special Sensor Microwave/Imager (SSM/I). Unusually low and stable microwave emissivities are observed in Oman and Egypt over large regions. Spatial patterns of low emissivities are well correlated with the distribution of tertiary sediments. Dielectric measurements have been performed and confirm that carbonate rocks have high permittivities (higher than 8 as compared to typical values of 5 for other rocks or 2.5 for silicate sands) and this explains the observed low emissivities.