Like ticndig says it looks like a flat button. Perhaps late 1700s to early or mid 1800s.
You mentioned that you can't make out the lettering on the back? Nick
Richard Robinson was established in Attleboro, Massachusetts in 1813.
He went into partnership with William Henry Jones in 1828, operating as Robinsons, Jones & Company until 1834. After a couple of name changes, the business was taken over by D. Evans c.1848