15 white-hot designs that prove platinum is worth every pennyRoses Are White Rose platinum necklace and pendant with brilliant-cut diamonds; $28,799; Heinz Mayer, Idar-Oberstein, Germany; 49-6781-9640; heinzmayer.deCOUTURE DESIGN Cosmic CurvesCosmo platinum brooch-pendant with shell, rubies, pink brilliant-cut diamonds, and 18k red gold; $16,077; Atelier Zobel, Constance, Germany; 49-75-312-5962; atelierzobel.com City SlickerArchitectural ring in platinum with diamonds and gemstones; price on request; Tournaire, Paris; 33-4020-0019; philippetournaire.com Geometry of LoveArt Deco 17-inch platinum and diamond sautoir with a 3-inch pendant, circa 1928, by Tiffany & Co., with 10.04 ct. pear-shaped diamond; price on request; Siegelson, New York City; 212-832-2666; siegelson.com FASHION Infinite BestMoltissimo platinum pendant with brilliant-cut diamonds on a leather br