When it comes to diamonds, a little goes a long way. Set with eye-catching cuts like baguette and marquise, these delicate designs don’t skimp on sparkle.            Baguettes IN THE OPEN Open ring in 18k gold with 1 ct. t.w. baguette-cut diamonds; $3,600; Mary Esses, NYC; 917-941-0826; maryesses.com FEATHER FORECAST 10k yellow gold long feather cuff with 1.85 cts. t.w. baguette diamonds; $4,600; Luis Morais at Muse, NYC; 212-463-7950; luismorais.com     HANG RELATED Necklace in 18k gold with 0.55 ct. t.w. baguette-cut diamonds; $4,000; Finn, NYC; 212-929-9556; finnjewelry.com   Celestial Motifs CRESCENT PRETTY 18k gold necklace with 0.95 ct. t.w. diamonds in enclosed double-sided white sapphire dome with 0.38 ct. t.w. emeralds; $6,280; Moritz Glik, NYC; 212-679-09