Lady Diana Spencer got engaged to Prince Charles in February 1981, and he gave her a ring with 14 diamonds circling an 18 carat oval sapphire.
There are lots of stories about how the ring was chosen, including one that Diana simply chose the biggest ring on the tray. However the real story is more romantic.
When Prince Albert got engaged to Queen Victoria in 1840, he gave her a sapphire brooch surrounded by diamonds (in those days they didn't exchange rings). Victoria and Albert had a famously happy marriage, and when Prince Charles chose the sapphire ring for Diana from the jeweller Garrard, he was hoping his marriage would be equally happy.
When Princess Diana died, the ring was inherited by Prince William who gave it to Kate Middleton in 2011. Their marriage appears to be as happy as Victoria and Alberts.