Alright well here is my argument for an against the first ring..
-Unique/antique looking setting.. I have never encountered a ring set like that and it doesn't have that "junky" look. Doesn't mean gold, but could be antique gold filled.
-You say the opal is real, any ring where the stone(s) are real is a sign in the right direction.
-The back cut of the ruby looking stone looks strange to me, but I'm not super familiar with how they cut them in a ring like this.. See the 2nd and and 3rd picture, looks almost like it was rounded off? Again, an unknown for me is neutral vote, because I haven't seen costume like this either.
Now the final observations..
-Inside the ring, there appears to be a few scratches which show no discoloration.. good sign for gold. (Both that it can be scratched seemingly easily and that there is no coloration)
-Picture 2.. The inside of the ring on the right side clearly has a marking does it not? Possibly with a portion worn off? Can't quite tell. If it's worn, my vote is gold.
From everything I've seen from the pictures I'd say it is gold. The good thing about it is - even if it isn't gold - it is a nice, antique/unique looking, possibly gold filled, costume ring. And I could probably get my $5 back out of it honestly and would buy without worry.
The second ring I won't comment much on because I can't see enough from the pictures, but here are my observations.
-The design of the ring puts it in a right direction, but not to say that costume jewelry couldn't have this.
-The stone appears to be a simple fake (I'm not gemologist just how it looks from the photo to me)
-I cannot see the inside of the band and the lighting is somewhat dark for me to be able to see the color of the entire ring.
From the looks of it, I'm leaning away from gold just going off of the pictures. I would say obviously worst case scenario a nice costume ring, which again you could probably get your $5 back out of. Due to the lighting in the photos/discoloration I'd have to say it appears to more likely be possibly rolled gold/gold filled or complete costume..Not saying it can't be real, if it is its likely a low karat 10 or 9k and the stone I still think is probably fake. Either way I'd have definitely inspected this ring carefully myself and may have even walked away with it not knowing for sure.