Those sexy ’70s clothes slinking across the fall runways begged for long, lean jewels, so it’s no surprise the Y necklace was all over Vegas this summer. Dubbed the bikini necklace by some (because it lands atop the breastbone), this slender chain-plus-drop style first came to the forefront in the early 1990s. But it’s back, ready to be layered with station necklaces or—yet another ’90s design—chokers.       Y necklace in 14k yellow gold with a 1.35 ct. green tourmaline and 0.22 ct. t.w. diamonds; $2,500; Jacquie Aiche, Beverly Hills, Calif.; 310-550-7529, ext. 101; jacquieaiche.com   Twin Drops necklace in 18k yellow gold with sweetwater pearls; $1,805; Efva Attling, NYC; 212-510-7072; efvaattling.com     Double Diamond lariat necklace in 14k rose gold with 0.13 ct. t.w. diamonds; $850; EF Coll