I think to better clarify with the last Emerald on the bottom right next to the credits, that is actually an breakable object used in the then unused Hidden Palace Zone in Sonic the Hedgehog 2. It has no significance to the overall plot other than to, I think, block several chutes that Sonic or Tails can enter to get to other areas of the stage.
Quick update: The video I found didn't have the breakable emeralds, so I replaced it with a different version that had them.
This video better clarifies what I mean
Quick update: The video I found didn't have the breakable emeralds, so I replaced it with a different version that had them.