This rectangular stone, introduced in the seventies, lives up to its name: By combining long, lean, step-cut and triangular facets, it refracts lots of light. So if you decide on this shape, keep the setting simple. Heidi Klum has a radiant-cut yellow diamond from Seal.

1.76-carat radiant-cut diamond with pave accents in 18kt white gold, Blue Nile, $12,300; 88-565-7641 or bluenile.com.