Did I buy this one before? I'm worried about what that would say about me...

Okay, I guess I just mentioned it before. Whew! From 1973, Superman #261, "Slave of Star Sapphire!" Story by Cary Bates, pencils by Curt Swan, inks by Murphy Anderson. 

Old Superman stories are probably not considered to have strong plots, but this one isn't just doing "let's you and him fight," it's going the long way around. The Star Sapphire--the stone itself, not the lady--is on loan to Metropolis, so of course Lois Lane is writing a story about it: Carol Ferris wasn't publicly known as Star Sapphire, so she was a bit of a mystery. The museum curator also has a replica of Star Sapphire's uniform, and suggests Lois wear it--purely for research purposes, of course. He even says Lois and Sapphire look alike: is this a subtle dig at DC, that the heroes' girlfriends are basically interchangeable? Nah, they aren't that self-aware or meta. 

Meanwhile, Clark Kent has to make an elaborate exit from a helicopter-shuttle (which had a very elaborate escape system for its passengers, that was probably immediately scrapped after Kent 'accidentally' sets it off) because a local mobster has a suicide-vest full of nitro...for reasons. I think his reasoning is, the cops and Superman can't do anything to him, or he'll explode and probably take out a city block or two. He's also holed up in the Green Lantern tavern, which actually appeared before, in Action #317! But, another passenger on the helicopter is watching this situation: Carol Ferris! Seeing the 'Green Lantern' seems to trigger something in her, which is probably exasperated when Superman blows up the bar--somewhat literally, with his super-breath. That doesn't hurt the mobster, and seems like a lot of property damage, but it lets Carol watch the scene with binoculars. Only, she sees it as Superman killing Green Lantern, which changes her persona to Star Sapphire's, seeking justice. She needed her sapphire first, though.

Back at the museum, the curator realizes he's accidentally given Lois the real Star Sapphire. Almost like you were distracted by something, huh? At GBS, Lois gives Clark a momentary hassle about his helicopter ejection and Superman showing up, but she had been burned so many times when she suspected Clark was Superman that she's given up on that one. When Lois won't tell what's in her bag after she drops it, Clark uses his x-ray vision, and sees the Star Sapphire outfit. Time for some fun! Or mean-spirited revenge, either or. Downstairs, Carol is storming the building, homing in on her gem.

Clark causes Lois's electronic desk to 'malfunction,' and 'finds' the Star Sapphire outfit. Why, Lois, you aren't really--Lois indignantly protests, while Clark gets to enjoy the shoe being on the other foot for once, until the Sapphire starts spinning in mid-air! It returns to Carol, who transforms into Star Sapphire before Clark gets a look at her. Then, she zaps Clark, launching him into the elevator shaft and nearly taking out a full car of passengers. Sapphire goes outside and starts wrecking things, to draw out Superman. 

Supes is pretty sure Green Lantern isn't dead, but also can't bring himself to punch Sapphire out, either. That gives her the chance to get the gem around Superman's neck--but still under her control! And it works pretty well, as she forbids Superman from removing the gem, or opening his mouth, and forces the cover scene, to kiss her boots! Still, using super-ventriloquism, Supes is able to call Lois, who still has the replica costume...

Sapphire orders Superman to destroy the Galaxy building and everyone in it, and he can't do a thing to stop himself--except, another Sapphire orders him not to! Lois orders Superman to fly up, knowing the real Sapphire would counter that and order him to fly down, causing the gem to fall off. Recovering the gem, Star Sapphire escapes, swearing vengeance...until the next day, when she sees a newspaper story with Green Lantern alive, and reverts back to Carol. Superman thanks Lois for saving him, but she asks him not to tell Clark, he already thinks she's Star Sapphire, and she'd never hear the end of it.

EDIT: It occurred to me later, I have no idea how well this fits in with Carol/Sapphire lore from Green Lantern, since this makes it seem like the gem had the power, true, but the Sapphire persona was inside Carol somewhere. I'm not positive that was always the case, though.