Backwoodsbob, in my 40+ years as a civil war relic digger and dealer, mostly in the Richmond-to-Fredericksburg (VA) area, I've seen several dug examples of your button, definitely from the civil war era, gold-plated, with an iron back. The pattern of the design is distinctive enough that I recognize it. It is indeed a Civilian-usage button, but for whatever my humble opinion is worth, it absolutely is not a 20th-Century button.
Civil war relic diggers and collectors call buttons with similar patterns a "flower button" because so many do have a floral design, but (like yours) some are more "geometric" in their design than floral.