BOOK REVIEW / Ringing the changes down the centuries: Rings - Diana Scarisbrick: Thames & Hudson, pounds 32

MADAME DU CHATELET died with a ring on her finger. It was a secret picture- ring, containing a hidden portrait. Voltaire, whose lover she had been, asked the valet to bring it to him, thinking that it concealed his likeness, but, sadly for his amour propre, he had been supplanted. Casanova was on surer ground. His portrait was hidden under a devotional image of St Catherine and worn by his mistress who lived in a nunnery and (probably) stayed faithful to him.

Casanova knew about rings. He had several and considered them to be part of the reason that he was widely regarded as 'un personnage imposant'. One of his collection, a portrait of Louis XV, was noticed, passed round a London dinner table and held to be an excellent likeness, though who can be sure how many people actually knew what the French king looked like? Like the clandestine Bonnie Prince Charlie rings, or those circulated while Napoleon was in exile bearing the pious hope 'Il reviendra', the message was on the finger.

This is a huge and handsome book. Only a true ascetic could resist the appeal of these fabulous baubles. The photographs show light glinting and sparkling off the writhen hoops, striking fire from the niello bezels, picking out the details of the encrusted intaglios and smouldering in the simple cabochons. I held out through the dark and middle ages, survived the fabulous creations of Renaissance Italy, weakened at the description of the Duchess of Somerset's 'little signet of gold with her Grace's own crest a castle therefrom issuant a demilyon crowned and holding in his arms a fireball', and completely collapsed at the sight of Mary of Modena's wedding ring - a simple little gold chain studded with five faceted rubies, so wearable. Lucky Duke of Norfolk to own it now.

Even without Wagner and Tolkien, rings have always been magical. Clever medieval marketing sold narwhal tusk as unicorn horn and fossilised fish as toadstone, both potent charms; later, pictures of saints on a finger were thought to protect the wearer - St Vitus against epilepsy, St Barbara against sudden death and, oddly, two of the three wise men against cramp. Perhaps they did. On the other hand, or maybe on the same, the Duke of Monmouth believed that a stone hidden in his ring would safeguard him in battle, which it certainly did not.

Diana Scarisbrick's text suggests that she knows a great deal more than is published here. Her information is condensed and tantalising. She mentions that Pepys whiled away some time while the lamb was roasting by thinking up a message or 'posy' to engrave in his cousin's wedding ring. Yet, to be fair, if curiosity prompts you to see what he came up with, you'll find that he, too, fails to finish the story (and, incidentally, it was mutton, and it was stewing).

Her introduction gallops through thousands of years, from the Sumerian civilisation through Egypt, the Bible and the classics until it skeeters to a halt as the first chapter opens in the Middle Ages and a Venetian knuckle-duster appears in the margin, crowned by a sharp, raised, revolving spur.

It is when she allows herself breathing space that her book becomes fascinating. Stories of the individuals who owned and exchanged these tokens of love and power give life to the objects themselves. These are people such as Margaret Paston asking her husband to wear the picture of St Margaret she had sent him 'for a remaumbrance tyle ye come home'; Nelson giving Emma Hamilton a 'fede' or clasped-hands ring; Prince Albert presenting Victoria with an emerald-studded snake; Oscar Wilde giving himself a sphinx.

Most surprising is how practical rings can be. The earliest watch comes from the 1590s, complete with tiny hour-striking mechanism, and about the same time was invented the horrible squirt-ring with attached syringe. There is no mention of the Borgias' poisoning finger-equipment, but there is a useful card-sharper's ring, with a polished perspective on one's neighbour's hand. Could this be why so many bejewelled old ladies are unbeatable at bridge?