Re: Could this be civil war era?
You found the front portion of a civil war era 3-piece US Army Staff Officer's button.
In case you don't already know, this type is called a 3-piece button because it has 3 main-body components ...the front, the back, and a rim-ring which held the front and back together. Note, the metal loop for sewing it onto cloth is not counted as a main-body piece.
This type of US Army Staff Officer button (with basically the same emblem as yours) was in service from 1832 until 1902. I've dated yours to the civil war era by several factors, which include its high-convex front, the shape of the shield, and the very-narrow rim around the edge of the front. The earliest version of your button (1830s) had a lower convexity front than yours has, and the post-civil-war version usually had a wider rim-ring than yours had.