When this 18k gold Rolex hit the auction block at the Christie’s Important Watches sale in New York City in December, the auction house described it as “superlative” and “possibly unique,” but true collectors needed to hear only one word before their interest was piqued: cloisonné.

“Cloisonné dials are rare in general, and even more rare when in perfect unrepaired condition, due to their fragility,” says Jeffrey P. Hess, coauthor of The Best of Time: Rolex Wristwatches—An Unauthorized History. The ancient metalwork decorating technique—which uses vitreous enamel to build elaborate, delicate scenes (in this case, a map of the Americas)—is so rare that Hess breaks a steadfast rule to recommend them to collectors.

“Over the years, I have been asked thousands of times for advice on investing in watches,” he says. “Typically, I shy away from specific advice. But when pressed, I always suggest three segments. On the entry level, pocket watches of any kind and 1950s to 1980s Rolex sport watches. At the expert level, one thing only: cloisonné dials. These dials are the crowning glory of any collection.”