Industrial designer Ken Goldman recently sent us pictures of "Spark," a pair of his-and-hers rings that can be struck to literally spark a flame. The flint is cut from a commercially-available flint from "one of the popular suppliers": if struck on "any sharp surface, it will shed sparks—with the right tinder, it will start a fire." The steel is good candidate, though Goldman admits that it would work better if it were even sharper.
Both are set in a sterling silver ring for handy access, not to mention potentially combustible symbolism... it's definitely a huge improvement on those cheesy 'broken heart' charms.
I'm not sure if the rings would function as a firestarter when the lucky couple is actually wearing them... though if they're particularly well-cut, I could see the pound becoming a potentially dangerous form of greeting.
He's currently seeking a manufacturer for the set of rings, among various other designs. In the meantime, I'd suggest that he comes up with some more eye-catching product shots... or better yet, a video of the rings in action...