I agree with Trk5capt that it's civilian, and it could very well be of, or near, the period. Without any obvious attachment points on the reverse, it's hard to say exactly how it might have been used; but unless it was worn on a dress belt or sash, my guess is that it's an ornament from some sort of larger object— trunk, furniture, frame, etc.
It appears to depict the Roman goddess of war, Minerva, who is often shown wearing a helmet. Usually, there is a dragon atop the helmet, as in this case, although some are so stylized that you really can't tell what they are. Images from classical mythology were quite popular in the mid to late 19th century, and Minerva seems to have been a special favorite. For example, here's a link to a page of Minerva buttons:
http://www.iwantbuttons.com/trays/WRBA/2003/image40.htm
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