Random close-packed materials are abundant in nature and industry, yet still poorly understood. Formed by random forces, these materials are often naively assumed to be as disordered as the common liquids, where all spatial directions are equivalent. Yet, the mechanical stability of random close-packed materials under gravity, implies spreading of a network of mechanical forces across a macroscopic sample; thus, the rotational symmetry may be broken, and the correlations along the direction of gravity may extend to a longer range. Unfortunately, direct real-space experimental measurements of structure and correlations in random close-packed systems of simple spheres are still missing; this significantly limits the physical understanding of these materials.

We form a random close-packed material by sedimentation of colloids from a homogeneous suspension and measure the structure of this material by confocal microscopy. Strikingly, the structure of our sediments breaks the rotational symmetry; thus, these sediments are more ordered than the common liquids. To quantify the excess order in these systems, we measure the density correlation functions along the different spatial directions, and compare the decay length and the period of oscillations to numerical estimates. Unintuitively, the correlations along the direction of gravity extend to a shorter range, compared to these in the perpendicular direction; moreover, both types of correlations are shorter than theoretically predicted.