Sparkling Po Engagement Ring*

Add sparkle to your sapphire ring by sandwiching a thin disk of Polonium-210 between the stone and the gold setting. This radioactive isotope decays to produce an alpha particle. When the alpha particle passes through the gem, it will produce an effect called scintillation - as each particle collides with the material of the gem, it produces a tiny flash of light. These flashes of light are strong enough to be seen individually by the unaided eye. In a dim light, your ring will seem to sparkle from within, like a sky full of shooting stars.

Of course, no matter how pretty it looks, no-one's going to wear it if it irradiates their finger. That's why I specified polonium-210. It has several properties that make it ideal for this use:
1) The only radioactivity it produces is alpha particles. These can be stopped by a thin sheet of paper. The chances of them penetrating a gold ring are about the same as the chances of you falling off a chair and ending up in Australia. (If you are already in Australia, please substitute appropriate far-side-of-the-world place)
2) The decay product of Po-210 is lead, which is not radioactive at all, so there's no danger of less benign daughter isotopes producing nasty hard-to-stop beta- or gamma-rays.

Finally, ok, I know there is a similar idea posted. [Link] But the aesthetic effect of this is completely different: instead of a constant uniform glow, you get an animated sparkle. I think this would be ideal for engagement rings: the half life of polonium would mean that the sparkle would fade away after a year or so, providing a nice visual analogy to marriage.

*Used to be Scintillating Gemstones. Renamed in honour of [Po]