.99 Mid-Century Marquise Diamond Ring in 18k Yellow Gold and Platinum. 99 Mid Century Marquise Diamond Ring in 18k Yellow Gold and Platinum. This fabulous Mid Century ring features a . 99 carat marquise cut diamond set in 18 karat yellow gold and platinum. The chunky marquise is wrapped in a halo of channel set emeralds, and also a halo of round diamonds. The emerald accents are set in a channel of milgrained yellow gold with a slight gap of open space separating it from the center stone. The shank of this ring is made