The Hope diamond, est. US$250 million. At the National Museum of Natural History in Washington D.C. The colour is suppose to be blue, but the camera lighting skewed it. You can read about its long and dramatic history from this link. The centre diamond is a natural blue stone around 45 carats, there are 16 white diamonds surrounding it with 45 in total for the whole necklace. From what I've read, the blue diamond is the rarest of all natural diamonds and this particular gem is the biggest of them all.

Speaking of diamonds. This company sells artificial diamonds of a different kind. They've been around for a few years and I've seen them on the news a couple of times. At first I was skeptical, but they seem legitimate and is actually doing well (seeing that they're still around after some time).