A crown gets an afternoon. A ring gets a lifetime, and the courts of Europe understood that arithmetic long before anyone photographed a wedding. The smallest object in a royal vault has always carried the most weight, which is how a gold band ends up standing in for a treaty, a dynasty, and a receipt.

The circle does the first half of the work: no beginning and no end, so it says forever without speaking. Egyptian couples braided bands of reed, Roman brides wore plain iron, and Roman writers insisted the fourth finger carried a vein straight to the heart. Add the gem and the ring becomes a signature, pressed into hot wax on a treaty. Gimmel rings split into two or three bands so each betrothed could keep a piece until the wedding locked them together again. Queen Victoria's coronation ring was sized for the wrong finger and iced off her hand afterwards. Later came the mourning rings: black enamel, jet, a lock of a dead man's hair under glass, and words cut inside the band where only the wearer's skin would ever read them.

Sapphire sits at 9 on the Mohs scale, harder than everything except diamond, so a blue stone can spend 400 years on a working hand and still look carved yesterday. Napoleon gave Josephine a toi et moi ring in 1796. In 1981, a prince put a 12-carat Ceylon sapphire ringed by 14 diamonds in 18-carat white gold on the hand of a 19-year-old, and the part that unsettled the court was the tray it came off: Garrard had built that ring months earlier, and any customer could have ordered the identical one for about $28,000. She wore it for 16 years, it stayed in a vault for 13 more, and in 2010 a second prince carried it to Kenya for a second bride. Along the way, stones themselves became the inheritance, lifted out of a mother's tiara or a collection nobody else was allowed to touch, and set again rather than replaced. Then look at the underside of any band worn for decades. The gold there is thinner than the front, because a thumb keeps turning it while the mind is somewhere else.