This spring, Van Cleef & Arpels’s High Jewelry collection is introducing the new Ruban Mystérieux watch, an angular, bracelet-like cuff with bands of snow-set diamonds edged in brilliant sapphires and emeralds, suggesting what the French call a ruban, or ribbon. A 3.72-carat oval-cut diamond covers the dial, and green and cobalt gems are invisibly held in place by the brand’s signature Mystery Setting, which was patented in 1933. Requiring nearly 1,300 hours to produce, the timepiece is a feat of craftsmanship — an almost diaphanous objet that belies all the precision and expertise it took to make it.