Wow, I just saw the episode "Blue Butterfly" of Castle this evening, and started searching for a necklace that looked similar. They're eBaying the actual necklace for charity!
Here it is, ending on Valentine's Day! It's currently at $2k with 50+ bids and 6d15h left. Good lord I wonder what it'll end up at! This is a great chance to snag some close up photos of the prop for any future group projects to recreate it.
Anyway, awesome prop. There aren't many Castle-specific props out there. As an aside, I got to see the actual steampunk arm pieces used in the episode on display at a convention at the artist's booth.