Elegant: Tiered, delicate necklaces. Beautiful with solids, but won't necessarily agree with patterns. Appropriate for nighttime events and formal functions.
Proper: Anything with pearls. Always appropriate (see below).
Bold: Colorful, bubbly necklaces. Appropriate for daytime functions and more casual nighttime events. In my opinion, the obnoxious resin neon bubble necklaces can cross over into tacky territory, but not necessarily. A good rule of thumb: If you think it might be tacky, it probably is.
Girly: Floral necklaces. The rose ones have become very popular, but there are so many varieties. These are appropriate for daytime events and can be paired with casual outfits.
Nautical: I love the coral reef necklace pictured above. It would be perfect with a royal blue dress, but could be dressed down even more for a daytime function.
And when in doubt, remember the wise words of Jacqueline Kennedy: