It is the US Army "Great Seal" 1902 - Present, and it is a belt buckle not a hat pin... exactly when? I dunno... Can't research it right now, but if nobody else replies I'll get more information later.
Are you sure its not a front of a two piece button? I just recovered two of them the same size of that you have. One had the back still and the other did not. The one without the back looks very much like yours. If it was a button, it was manafactured by Art-Metal Works Inc (Ronson) between 1910 and 1930.
Thanks for the info PBK. Its just so small... The two buttons I dug the other day are slightly large than a quarter so size wise they match up... Went back yesterday and dug a matching cuff.