Your pendant, which shows a ship, is definitely not from the civil war. Your pendant's ship is a slightly incorrect representation of the US Navy battleship USS Maine... which blew up and sank in the harbor at Havana Cuba on February 15th 1898, prompting the Spanish-American War a bit later that year. The US battle-cry in that war was "Remember the Maine!" Your pendant is a patriotic memento of that time.
https://learning.blogs.nytimes.com/...s-battleship-maine-explodes-in-havana-harbor/
Your pendant's rendition of the USS Maine is incorrect because the Maine had only two smokestacks, not three as seen on your pendant.