Sapphire and Diamond Ring. Sapphire and diamond cluster rings are immensely popular as a choice for engagement rings. Most famously worn by Princess Diana (whose engagement ring now adorns the Duchess of Cambridge), they’re classic, sophisticated and elegant. This beautiful early 20th century example is set with a cushion-shaped sapphire in an attractive deep blue, within a ring of early 20th century transitional circular-cut diamonds, to a band of yellow gold - the perfect choice for any bride-to-be.