A few weeks back I was thinking of ways to fuse electronics with jewellery. The first one I came up with was an LED on diamond. You could have a classical diamond ring with an LED built in the bulk. Gold contacts could be evaporated on, and could merge in nicely with the rest of the ring!
Diamond occurs naturally both intrinsic and p-type. Blue diamonds, which do turn up in the ground, are n-type due to boron doping. N-type diamond does not occur naturally, but several dopants have been suggested. Later, growth of phosphorus doped n-type diamond by PECVD was demonstrated, then the same group made a p-n junction in diamond and it worked as an LED . All pretty awesome stuff. There's a minor problem - the bandgap of diamond is pretty wide ~5.5eV. That's pretty short wave UV - in the germicidal region. Not ideal for jewellery! There are solutions to that, though - white LEDs are made with blue or UV emitters coated in a mixture of "phosphors" which fluoresce to make wideband, white, light.
So let's start off with a nice, natural blue diamond p-type bulk, and grow a small slab of n on top with PECVD. Evaporate on gold, then pattern by photolithography to make contacts, take one out each side to blend in nicely with a gold ring. Might also need to deposit some kind of passivation over the whole thing for mechanical and oxidative protection. Powering the thing is obviously going to be a challenge - especially in a ring. There are some pretty tiny Li-poly cells around these days, could probably hide one on the reverse. Alternatively, have a pendant with a tiny battery hidden at the back of the neck. In the clasp, ideally. Heaps of space in a bracelet.
Now, aesthetics - this clearly must be very carefully designed to remain tasteful. Adding LEDs to diamonds could easily end up as some horrid D&G chav nightmare! A collaboration with Moritz Waldemeyer, perhaps.
Interesting extensions to this idea are as numerous as they are obvious. The first is to try some different gemstones. Sapphire can also be doped, and is a widely-used substrate in optoelectronics (much less experimental than diamond). It's used widely in jewellery too, even large, relatively inexpensive synthetic things like scratch-resistant watch glasses. It's a good lattice match for GaN, which is already mass produced into blue LEDs. Think I prefer the sapphire/GaN to diamond, because you wouldn't need the phosphors and it would be the jewel itself that creating the light you see. SiC is also used for some optical things - is that a gemstone? What does it look like?
If growing onto natural, impure substrates with defects is too difficult, we could synthesise whole optojeweltronic devices from scratch - standard Czochralski boule growth, cut into whatever shape you like, CVD/implant/evaporate whatever on top, package, sell. New functionality might be a good selling point to boost synthetic stone sales.
Merging with other solid state electronics and sensors could add some cool functionality - glowing when it's hot/cold/humid / when your loved one is near/you get poked on facebook/insert marketing crap here.
Early devices would be obscenely expensive, but world firsts. Perhaps an application for Darren in the new ECS cleanroom?!
 Nitrogen and potential n-type dopants in diamond - Kajihara, S. A., Antonelli, A., Bernholc, J. and Car, R. - Physical Review Letters Vol. 66, No. 15 - April. 1991
 Phosphorus-doped chemical vapor deposition of diamond - Koizumi S., Teraji T. and Kanda, H. -Diamond and Related Materials Vol. 9, Issues 3-6, April 2000
 Ultraviolet Emission from a Diamond pn Junction - Koizumi S., Watanabe K., Hasegawa M. and Kanda H, -Science Vol. 292, No. 5523 - June 2001