[jtaillon]

Moissanite is also synthetic (typically grown via a chemical vapor deposition process). It's not carbon (like diamonds), but made of SiC, a material long used for abrasives and more recently for high-power electronics and LEDs. The 4H-SiC polymorph is actually extraordinarily rare in nature, even moreso than diamonds!

We chose moissanites as the primary stones in my wife's engagement rings and everyone talks about how pretty it is. It's a bit more "firey" or "sparkly" than a regular diamond (because of more birefringence), but I guarantee no one outside a very highly trained jeweler would know the difference (if that's important to you). It's worth it to spend the money on the higher quality ones though, since just like diamonds, the inexpensive ones will have yellowish or greenish tints. Even still, they're at least an order of magnitude cheaper than similar diamonds ($600 vs. $6,500 at my quick search for ~1 carat).