“Men, in general, have become more image-conscious,” said Maia Adams, co-founder of Adorn Insight, a market research firm in London focused on the jewelry trade. “It’s okay for them to follow fashion, groom themselves, look after their bodies, watch what they eat. Even the vogue for tattooing and piercing has become so mainstream that it’s unsurprising jewelry has become part of that self-expression box of tricks.”

The appetite for jewelry among millennial males is one oft-cited reason for the surge in interest in the men’s category. “It is well-noted that millennials have grown up in a more ‘accepting’ society, where things such as same-sex marriage and mixed-race relationships no longer turn heads,” Ms. Adams wrote in an email, “so it makes sense that men’s jewelry — once considered rather niche — is now increasingly mainstream.”

Research bears this out. Earlier this year, Noise/The Intelligence Group , a youth-focused marketing agency based in New York, released a report that found that, in a survey of 14- to 34-year-olds, 34 percent of men were willing to pay for a luxury accessory.

The agency’s chief marketing officer, Jamie Gutfreund, said the need to stand out in a demographic as large as Generation Y helped explain a penchant for rare and distinctive luxury items. “Millennials as a generation — there’s two billion of them around the world,” she said. “How will they differentiate themselves?”

There is nothing inherently new about men donning jewels to stand out in a crowd. From the gold chains that adorned ancient Sumerian rulers in Mesopotamia to the elaborate diamond necklaces beloved by India’s maharajas, jewelry was a man’s game from the very beginning — and the more powerful the man, the more sumptuous his ensemble.

“Where it probably splits, like everything else, is the French Revolution,” said Beatrice Behlen, senior curator of fashion and decorative arts at the Museum of London, whose recent “Tomfoolery” exhibit featured photographs of male Londoners wearing jewelry.