The image below was taken on a hike. Circular patterns at bottom (it is flatter on this face and round otherwise) appear either man-made or possibly it is a fossil.

Any idea what this could be?

Rock of interest:

  • $\begingroup$ Welcome to Earth Science StackExchange! Could you provide a bit more information? Please follow this guide: meta.earthscience.stackexchange.com/a/125/56. $\endgroup$
    – arkaia
    Jan 17, 2017 at 18:42
  • $\begingroup$ I don't suppose you have a picture of the chipped side. It's useful to see an unweathered surface when identifying rocks $\endgroup$
    – haresfur
    Jan 17, 2017 at 22:59
  • $\begingroup$ There look to be some radial markings in the centre which lead me to suspect that this is a fossil but I'm no expert on what it could be. $\endgroup$
    – bon
    Jan 18, 2017 at 8:09
  • 2
    $\begingroup$ OTOH From this single photo with lack of information, this could be concrete $\endgroup$
    – Jan Doggen
    Jan 18, 2017 at 10:59
  • $\begingroup$ It could be a concretion having a image of the other side and the broken surface would help a lot. Density and were it was found will help as well. If the other side is broken off you might have a fossil or at least a cast of one. $\endgroup$
    – John
    Jan 18, 2017 at 15:29

1 Answer 1


The face of the rock with the round center portion looks strongly like a Rugosa or Horn Coral. I suggest it is a Rugosa coral because of the center with the radial lines out from the center and the clear edge surrounding the radial lines. See https://en.wikipedia.org/wiki/Rugosa.

  • $\begingroup$ Thanks for the feedback. The distinct feature that made me suspect fossil was the radial lines out from the center as well, in which case Rugosa makes sense. $\endgroup$
    – DrTarr
    Jan 19, 2017 at 1:08

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