A jewel neckline is one that encircles the neck, sitting at the throat. I thought this Legends by Romona Keveza was a great example of one (although it is hard to tell from the mirror reflection in the picture below if this is more of a halter style as well.)
The jewel neckline features a gathered illusion overlay over a soft sweetheart bodice. The gown is finished by a clean lined, full skirt and a jeweled natural waistline.
Credits: Legends by Romona Keveza style no. L259