A flat pastel marble image cheats your vision long before you suspect any trick. On the screen, nothing bulges, nothing shines, yet the brain insists on glassy depth because it is wired to treat certain color and contrast patterns as physical surfaces rather than pigment.
Most decisive is the gradient. Soft shifts in luminance mimic the way diffuse reflection distributes light across a curved stone slab, and the visual cortex treats any smooth, monotonic change as evidence of three dimensional curvature governed by Lambertian reflectance. Very slight tints toward blue in recesses and warmer notes on apparent ridges echo atmospheric scattering and subsurface scattering, so a flat patch is reclassified as a lit volume, not a tile of color.
Just as important are edges. Where pastel veins meet their surroundings, artists push local contrast and sharpen spatial frequency at one side while blurring the other, a pattern the brain reads as a highlight kissing a crease. Orientation specific neurons in primary visual cortex lock onto those abrupt transitions, then higher areas infer gloss from the micro contrast profile known as the specular highlight. Even fake occlusion, where one vein appears to pass in front of another through a tiny value jump, recruits depth-from-edges circuitry that normally handles real objects.
What seals the illusion is lighting consistency. A faint, repeated bias toward brightness along one diagonal tells the brain there is a single virtual light source, so every gradient and vein snaps into a coherent object under stable illumination. Within that closed system of cues, the fact that the marble is only pixels never really gets a vote.