It is really quirky, just like Luna, and yet it's absolutely adorable. Besides, with my natural blond locks, my choice of Ravenclaw as my House, I often say that I am Luna when I cosplay as a Hogwarts student. Add to that the slimming factor of layers and you know I'm bound to love it!
Looking around the net, I've been able to find a few good pictures of this dress. The following one shows in detail the neckline frill, which makes it look very vintagey to me, as well as the rhinestones on the fabric, which I hadn't noticed before, and the earrings.Evanna Lynch as Luna Lovegood, Harry Potter and the Half-Blood Prince (2009)
Luna Lovegood's dress for Porfessor Slughorn's party
It is upon seeing that picture that I realized the dress was actually purple. I honestly thought it was silver. I can also say that I believe the outer layer to be made of Organza.
Now the only thing I couldn't find a picture of is the bracelet. In the Making-Of video where Evanna Lynch sits down with Costume Designer Jany Temine, they talk about it and they even show it to us. Apparently, Evanna made the beaded jewelry that Luna wears on screen. For the bracelet, she made a little rabbit, as that is the shape of Luna's Patronus.
Haven't seen that video? That's all right, I found it for you on YouTube.