The Short Answer
Choose by the setting first, then the shape. If the diamond sits high enough for a band to slide under it, a straight band usually sits close, whatever the shape. If the setting is low or wide, a straight band stops against it and leaves a small gap. A curved or notched band closes that gap.
By shape, round, Asscher, emerald and radiant rings pair most easily with a straight band. Ovals go either way. Pears and marquises, the longest shapes, most often take a curved or V-shaped band. By setting, halos, three stones and toi et moi rings are the most likely to need a curve.
A small gap is normal, and many people like it. In this photo, the oval wears a straight band and the pear a curved one.

























