This is Sephora by OPI Traffic Stopper Copper over Forbidden Fudge:
I got lucky when the Sephora by OPI closeout started - back in 2013, I think this was - and got in on it while there was still a lot of stock left. This is probably my favorite of those polishes (I think I bought nine in all, three of them top coats like this one). I especially like this over the dark brown because it shows up so well. It's pretty straightforward - just a bright copper glitter in various sizes. I have a bunch of copper polishes, but this is the only bright-as-a-penny copper glitter, I think. Since this one is discontinued, I think somebody needs to pick up on that and make another one of those.
Added: Here's what it looks like with nothing under it:
The base is more or less clear (the solid coppery-looking nail is a different polish, this is the one above that) so it would look odd alone, but you could probably find a number of different things that it would look good with. My favorite is still the brown undercoat above, though.