Hi! I read a fic awhile ago where Spike gets the gem. During the fight, Buffy can't remove the gem because it's stuck. The ring brings out Spike's 'champion' side. Eventually, Willow and Tara find out how to remove the ring and tell Spike.

Hi, I believe it could be Not Heads or Tails by bramcrackers, on EF and AO3.

Summary: Spike
Turns out the Gem of Amarra is more complicated than just a get-out-of-death-free card and as soon as I put the bloody thing on, I find myself compelled to play the white-hatted hero. And the damn thing is stuck on my finger. Sodding magic.
Buffy
Riley is perfect, right? Normal, cute, way into me. Not involved with anything that goes bump in the night. So yeah. Perfect.
Except I'm not so sure his white-picket-fence future is one I want. In fact, it kind of makes me ill thinking about it.
And Will's will-be-done spell, and the very naked things that happened during it? That did not help.
Riley
I can handle Buffy being the Slayer. Really.
But she's my future wife and I'm not letting a tacky Billy Idol wannabe of an HST drag her into the darkness.

Thank you!!!