We present Illumination Cue-driven densification for 3D Gaussian Splatting (ICGS). Effective densification should allocate more primitives to regions with high geometric complexity, but the underlying scene geometry is unavailable under image-space supervision alone. Hence, existing methods primarily rely on photometric signals such as view-space positional gradients, which can become weak once a primitive is locally stationary and may therefore under-densify regions containing fine geometric structures. To better drive densification, we leverage implicit structural cues reflected in illumination patterns. Our key observation is that the underlying 3D geometry is unknown, but its surface relief, orientation, and visibility relationships leave indirect traces in illumination patterns such as shading and shadows. The statistics of such patterns can reveal structural discrepancies in the representation of underlying scene structures. Our experiments show that our approach greatly improves rendering quality with fewer primitives.