You would have thought that a plunging neckline and all that exposed skin basically begs for some added sparkle, but this is generally not the case. The outfit makes the statement here and any type of necklace would just try and take away from that.
You are much better off to accessorise with earrings, bracelets and rings/hair jewelry.
Again, a lot of bare skin and therefore room to work with. You have the choice of going with a pendant/chain style or add a bolder chunky necklace.
A V-shaped neckline looks best with a necklace following the V-shape, like a pendant necklace or a Y-necklace, never a round choker. The length of the necklace depends on how deep the V-neck is and will probably be around the 45cm/18 inch mark.
For square necklines, the best compliment is a short choker. Either a chunky one, a pendant chain ensemble or a strand of pearls.
The boatneck is not easy to accentuate. Never wear anything shorter, but go for the longer (minimum 20", 50cm) style.
Long chains with pendants work nicely and so do body necklaces.
A draped neckline can only accommodate a short, dainty necklace if anything at all.
A round neckline is one of the easier styles to accessorize. If the neckline is very high, you will want to go for a long necklace (similar to the boatneck style). If the neckline is a bit lower, a short necklace/choker will look fantastic.
You can wear just about anything with this neckline. Shorter Necklaces or longer chains/body necklaces. Avoid anything that just hits the neckline.
Depending on shirt and occasion, you might want to go either formal or relaxed. You can leave a couple of buttons open at the neck and accessorize with a short necklace/choker/pendant chain or you can go for a longer necklace (60cm/24" or even longer) and wear it over the shirt.
You can take your pick to liven up the turtle neck or polo neck. Either a short chunky necklace or a long dangly one. This one is a perfect canvas for a statement piece.
Also see the following chart: