The emblem on the object is the "Great Seal" of the United States. According to the book on American Uniform Buttons by Alphaeus H. Albert, the US Great Seal emblem has been used on "US Government Officials and Diplomatic Service" buttons ever since the 1790s. That emblem has also been used on US Army buttons from 1902 onward.
Your disc-shaped unidentified object's "construction" is important for its ID. It has a smaller disc atop a post on the main disc's back, instead of a thread-loop. That means it is not a button. It can be either a collar-stud, or a lapel "badge," or a cufflink. That also means it is not Military. The Albert book indicates it is Government Official & Diplomatic Service.
About its age:
The marking on its back says "WARRANTED 14K GOLD PLATED." Warranted is British-speak for "guaranteed"... which indicates it is British-made. (Many early-1800s British-made brass 1-piece flatbuttons have a backmark saying "warranted.") Gold Karat-rating terms such as 14K are not seen until the latter-1800s. The use of the term "gold plated" also indicates it is from no earlier than the late-1800s. Prior to that time, in the backmarking on buttons and similar objects, goldplating was called "gilt."