The gold content is a good place to start. There are gold buyers who allegedly pay for .03 point stones. My wife tried to sell an old ring to the biggest god-diamond buyer. The ring had nice rubies and diamonds like Yours. His offer was low on the gold, and he offered $2 per Ruby and $3 per diamond. I added up the totalm and his offer was $20 more than the gold is worth. I talked her out of selling it
Those guys will offer $300 for a nice half carat stone, and slightly more than zero for the stones. I think You can figure slightly more than current price for the gold content. I would wait to sell it until gold gets up in the $1300+ range. I really think it' a beautiful ring, and as others have said, 18K gold is usually used on quality jewelry. If it was me, I'd give it to my wife or girlfriend if they don't care that it's second hand. Great find though. I'm pretty jealous!