It’s a fascinating crossroads as designers, manufacturers, and retailers race to unlock the new formula for engagement ring desirability. (image above:) Jewelry by Imagine Bridal includes oval three-stone plus halo semi-mount, $3,335; marquise halo semi-mount, $3,825; petite set hidden halo semi mount, $2,375; pear shape swan halo engagement ring, $2,925; round swan halo engagement ring, $2,775; oval three-row pavé hidden halo semi mount, $3,500. Fana14K white gold engagement ring semi-mount (center stone not included). $26,150(212) 719-3333, jadetrau.comJohn Apel18K rose gold engagement ring with center diamond (1 TCW) and pavé diamonds (0.33 TCW). $36,236(415) 989-9235, #105, yaeldesigns.comZahn-Z18K yellow gold ring with fancy brown Mughal cut diamond (1.32 TCW) and accent diamonds (0.50 TCW).