Scan the recent New York collections and one overarching motif comes into focus: There’s a new erogenous zone for jewelers—a woman’s neckline.Designer after designer sent out models accessorized with little more than a necklace. Behnaz Sarafpour had Venetian glass pendants; Anna Sui, chunky chain-link beauties. Marcia Patmos of M.Patmos went the earthy route with necklaces made of reclaimed wood from a Brooklyn warehouse. Timo Weiland took things to edgier terrain with a collection inspired by The Girl With the Dragon Tattoo star Rooney Mara. His models wore slim, industrial collars and chokers that were as provocative as they were sexy.Derek Lam went similarly urban, albeit in a more restrained way, in his collaboration with sculptor-cum-­jeweler Thomas Bliven. The latter, who previously worked with artist Jeff Koons, came up with the riff on ID bracelets, which became chic chain