26mm, This is a neat relic.  It is a "poker chip" which was made from an Infantry Officer "Eagle I" button. Some soldier made it from a Infantry button. It was  dug in Petersburg, VA, and I suppose that the boys had plenty of time to play cards that winter.   It is not stepped on, it is instead painstakingly flattened... so much so that I wonder if it may have been put on a train track to be run over by a passing train.  Boys will be boys!