This is almost certainly a reproduction. An XRF scan conducted on 5 February 2018 failed to indicate gold but found zinc and copper.
This object is a bronze copy of a gold medal presented to Cyrus Field in 1867. Field's daughter Isabella loaned the original to the Smithsonian from 1899 to 1903. Curator George C. Maynard described the original as "Four inches in diameter; edge 3/8 inch (.376) thick; weight twenty nine and fourteen-sixteenths ounces, avoirdupois. Inscription on reverse side; "By resolution of the Congress of the United States, March 2. 1867, to Cyrus W. Field, of New York, for his foresight, faith, and persistency in establishing Telegraphic Communication, by means of the Atlantic Telegraph, connecting the old with the new world.”
He identified the original medal as a “duplicate." He wrote, "The first medal struck was sent to the White House, to be presented to Mr. Field by the President [Andrew Johnson] and Field was notified to call for it. When he called the medal could not be found and, after some delay, another medal was struck and delivered to Field. Subsequently, upon clearing up the White House, for a change of administration the lost medal was found and given to Field. One medal is in the New York Museum of Fine Arts; this is the other."
Maynard wasn't certain which of the original medals was which. Isabella Field Judson wrote to Smithsonian secretary Langley on 25 July 1899: “I shall be glad to loan to the Institution the Gold Medal given by Congress to my father. I cannot say whether this one is the original or not, as there were two struck.”