Villagenut is correct... your find is a very rare version of Confederate army "CSA" button. More specifically, this is called the "CS over A" version. It was made in New Orleans just after Louisiana seceded from the Union, and thus is what Albert calls a Confederate "Local" button, meaning, manufactured in the wartime South. It is shown in the Albert button-book as button #CS-88. All known specimens are this 17mm "vest size."
We believe the CS-over-A buttons were worn by the cannoneers of a Louisiana State Militia artillery regiment called the Washington Artillery, because a good number of these extremely rare buttons have been dug in campsites known to have been occupied by the Washington Artillery. It is thought that the "A" is separated from the CS because it is intended to mean Confederate States Artillery.
Most of the excavated specimens have a "push" in the button's high-domed front. Yours has only a small push/dent, so count yourself lucky.
Because you found it in a field in Northern Illinois, I would have to guess it was a wartime souvenir brought home by an Illinois yankee soldier.
The CS-over-A buttons were originally goldplated. But being made in the wartime South, the goldplating is thinner than the double or triple-plated gold on yankee and British-made uniform buttons... and thus you rarely see a CS-over-A with any gold gilt remaining on it. See the photo below.
