This study focuses on unraveling the microphysical origins of the nonlinear elastic effects, which are pervasive in the Earth’s crust. Here, we examine the influence of grain shape and relative humidity (RH) on the elastic nonlinearity of granular assemblies made of spherical glass beads and angular sand particles. We find that their elastic nonlinearity is of the same order of magnitude. However, while the elastic nonlinearity of glass beads increases with RH, that of sand particles is rather RH independent. We attribute this difference to the angularity of sand particles; absorbed water on the spherical grains weakens the junctions making them more nonlinear, while no such effect occurs in sand due to grain interlocking. Additionally, for one of the nonlinear parameters that likely arises from shearing/partial slip of the grain junctions, we observe a sharp amplitude threshold in sand which is not observed in glass beads.