A doubloon is about the size of a dime. And that could be old enough to have been beaten up by the rocks over the years. Otherwise it might have been a medallion or pendant - possibly religious. It would be interesting to know how much it weighs.
looking at the picture above of it pictured with a dime, it may be possible that it is a US one dollar gold coin. looks as if the '1' is visible horizontal at the 3 o'clock position. compared to this picture I've added, the size looks about right
It certainly has the appearance of a Gold coin that has been well tumbled by the sea....good find and welcome to Treasurenet If you can try and get some better close up shots, it could lead to a ID
You might want to check and see if it's 14k, 18k or whatever to try and help identify. US 2 1/2 dollar gold coin is .900 fine and is 1 mm smaller than a penny and 1/10 mm larger than a dime. Original weight would be 4.18 grams.
Congratulations!! But it's hard to tell gold or not from the picture, in spite of the test is positive. The gold is always nice, i believe you. Well done!! Good luck next hunts.