The hydrogen nuclear magnetic resonance (NMR) spin-lattice relaxation rate (R1) of three fully hydrated natural sands was measured and compared to that for a fully hydrated synthetic sand as a function of applied magnetic field and particle diameter (d). R1 values increased with decreasing particle diameter for both natural and synthetic sands. The increase in R1 became greater with decreasing magnetic field strength. The field dependent R1 data allowed the prediction of R1 at the Earth's magnetic field (BEarth). Measurable R1 differences were observed between the three natural sands, as well as the natural and a synthetic sand. The difference in R1 of the natural sands can not be attributed to the geometric characteristics of the grains, as these were identical within experimental error. The R1 differences can be attributed to trace paramagnetic impurities in the grains which influence the surface relaxation component of R1. These results show that it is unwise to predict aquifer characteristics based on R1 values from nuclear magnetic resonance sounding without knowledge of the paramagnetic impurities in the sand grains. Furthermore, the results emphasize that laboratory measurements of R1 should be performed at BEarth.