OK so it's a chain, the links are fairly small and it seems to be well made. It has the stamp on the clasp so as far as I can tell it looks genuine but I'm not a jeweller so I couldn't really tell. I needed this as I bought my wife a pendant and the chain was to short.
Great product for the price, I was a little nervous that the clasp or links would not be strong enough (the pendant is not heavy but it was fairly expensive and a memento from our honeymoon so I would hate to lose it). So far so good, I've had a bit of a play with it and it feels good - strong but supple.