SmokeyTheCat is correct, as usual... civilian 1835-&-later button. More specifically, your find is what Antique button collectors call a "Golden Age button"... a reference to the period from about 1820 to 1860, when the manufacture of very-ornate goldplated (or silverplated) brass buttons reached its finest craftmanship. As such, more than just a few people collect them.
Also more specifically... the five indentions on your goldplated brass 2-piece button's front have distinct "facets" to make them shine with golden light like a faceted gemstone. Due to the goldplating (called "gilt" for these buttons), the smaller indentions (which were laboriously hand-engraved by a master craftsman) produced a golden glitter effect. You can see why some collectors are willing to pay a good price for well-preserved specimens which have a lot of the original gold gilt remaining on them.
Congratulations. I've been a metaldetecting relic-digger for about 40 years, and I've only dug a dozen or so "Golden Age buttons" as good as yours.