Hmm it sounds vaguely familiar.
Thrill of the Hunt by vampirische liebe might be the one. (Found on this site or Elysian Fields)
It has similar elements, such as Buffy bumping into Spike on the quad. They talk for a moment (and Spike fakes an american accent and gives her a false name and Buffy notices his cold hands) but he doesn't have the Gem inserted in his chest, nor does he leave her gifts. So there's a chance that it isn't the one your looking for.
If it is

if not

Sorry and I hope you find it.