Gold coins aside per instruction from the OP....a 20 cent piece would be difficult due to rarity. A coin I HAVE found which is VERY hard to find is a 3 cent nickel. Almost nobody would be coin hunting in that range, which is even lower than a regular nickel. HOW I found it might be handy for the next guy to know, as that’s how a gold coin might also be found. I dug a signal with my IDXPro that came in at zinc. The 3CN was in with a modern quarter. I feel the best chance I have at a gold coin will be a similar situation, given the public sites and fairgrounds that I tend to gravitate toward.
The coin I have NOT found which I thought I would’ve found by now is indeed a TRIME. Being that I live in southern Wisconsin and we just don’t have much history here earlier than the 1840’s, it’s not easy to ”go old”. In fact, my oldest coin is an 1844 LC. That’s TOUGH to do! My number one bucket lister is a Seated half, followed by a trime and Capped anything.