I hiked the Lanikai Pillbox trail (Oahu, HI). I noticed eroded concentric rings in the igneous rock with a hard center. I'm unable to find the geologic cause of these rings. Are they Liesegang rings? There were several instances (~20) I observed on the hike.
Not knowing the setting and judging from the image alone in this trampled environment, I think we're looking at a form of chemical weathering called "spheroidal weathering".
The gist is this (from the linked wiki page): Penetrating water alters the bed-rock along cracks or joints, causing volume changes between altered and unaltered parts. These differences result in the formation of spheroidal layers that are removed by erosion (or trampling), leaving a central relatively unaltered boulder/pebble.
Liesegang rings are somewhat enigmatic, not sure if the term is applicable here.