Sounds like you're describing these tritium fobs. I have a couple, and they live up to the claims. A little pricey at $20 apiece, but unlike similar items that are GITD-based (glow-in-the-dark), these don't need to be "charged" with a light source, and they have a useful life of ~10 years. Have one of mine on my keyring and it's survived a pretty rough and tumble existence.

Coincidentally, I discovered these GITD-based key fobs a few days ago, and just ordered ten of them (some for me, some for my brothers). As you read the thread, you'll see that they do have to be "charged" by a light source, but the "glow" lasts for several hours — and they're considerably less expensive at $3/apiece for any of the three sizes. The feedback from those who've bought them is uniformly good, both as to build quality and performance.

//No affiliation with either of the two aforementioned sellers; just satisfied/hope to be satisfied with their wares//