It’s more likely a surface-lighting effect than chromatic aberration.

At very high magnification, a coin’s reflective, irregular metal surface can show strong tonal and color variations when lit from a low angle. That kind of side lighting emphasizes tiny scratches, relief, and texture, producing an appearance similar to what you’d see under a microscope.

Chromatic aberration usually shows up as consistent color fringing along high-contrast edges caused by the lens. What you’re describing on a burnished metallic surface is more consistent with the way the light is interacting with the coin’s fine texture than with lens CA. ISO 100 on the D5100 also makes image noise a less likely explanation.

So based on the setup and appearance, this is most likely the coin surface and illumination, not optical chromatic aberration.