The main attraction of any diamond is its eye-catching sparkle. Or is it?

Some designers have been turning that notion on its head by focusing on diamonds largely devoid of the brilliance and fire commonly expected of the stones. Portrait-cut diamonds — thin, flat stones that are minimally faceted along their perimeters and have a transparent glasslike appearance — are gaining a following among a clutch of designers and clients drawn to their unusual allure.

“The portrait cut, also called a lasque, is among the oldest forms of diamond cutting,” said Beth Bernstein, author of “The Modern Guide to Antique Jewellery.” “It originated in ancient India and got its name from the practice of placing portraits beneath the gems to enhance their appearance and protect them.”

Later, the same technique was used in Europe. “Royals and aristocrats would commission jewels with portraits set under thin slivers of diamonds, whether as status symbols or tokens of love and loyalty,” she said.

Vishal Kothari has devoted an entire collection to portrait-cut diamonds for VAK, his Mumbai-based jewelry brand that reinterprets motifs from Indian art and architecture with a modern eye. The style’s Indian origins first attracted him, and he said he “gradually fell more in love” when he discovered that the shallow profile allowed him to achieve an effect he couldn’t create with other diamonds.

“Because the stones are flat, I can use a thin wire, making them look like they are floating against the skin,” he said. “You see very little metal in my designs.”

He has applied that nearly invisible setting technique to diamonds in every tier of his collection, from high jewelry designs, like floral earrings composed of clusters of triangular diamonds ($42,000), to dainty studs ($2,600), often combining them with accents in colored stones — ruby, spinel, sapphire — or rose-cut and full-cut diamonds.

The novelty of the cut and its subtle character appeals to many of his clients, he said, including members of several royal families that have commissioned pieces with portrait-cut stones. And a diamond choker that converts to a tennis bracelet has become one of his best sellers. “Everybody has a full-cut diamond tennis bracelet. You can find it anywhere online,” he said. “But when it is done with portrait cuts it’s a bit cooler, and you can wear it anywhere, like the Tube in London, without attracting attention.”

Anup Jogani, a Los Angeles gem dealer who works primarily with high-end designers and specializes in rare, untreated stones, said he had witnessed a surge in requests for portrait-cut diamonds, including modern stones and antique diamonds taken from sources like watches. He considers them to be a connoisseur’s choice: “It’s a diamond for a diamond’s sake, not flash and sparkle.”

Prospective buyers, however, shouldn’t expect a bargain just because portrait-cut diamonds aren’t well known; they are “around the same price” as their highly faceted counterparts, Mr. Jogani said, though the slim profile of the stones give “a much bigger look per carat.”

With their increasing popularity, more options in portrait-cut diamonds are coming to market. Grace Lavarro, founder of Jewels by Grace, a retailer that offers vintage, antique and its own contemporary pieces online and in its Los Angeles atelier, said she had noted that more variety was available: “Contemporary portrait cuts come in really cool, funky shapes — kites and hexagons and cushions — whereas most antique ones would be slightly irregular in silhouette.” And while many antique stones had a brown cast, she said, “you will find a higher color, like D, E and F, in contemporary stones.”

Ms, Lavarro said one of her prime considerations in selecting a portrait-cut gem was whether a stone was free of inclusions, or imperfections, because the cut reveals virtually every flaw at a glance. But that crystal clear visibility makes such gems good at keeping mini-mementos within view. For example, a recent Jewels by Grace locket ring featuring a portrait diamond that can slide upward so items can be added or removed with ease.

Yoram Finkelstein, the founder and owner of GemConcepts, a diamond cutting facility in Ramat Gan, Israel, has worked extensively with diamonds using cuts that were developed in the past, including portrait-cut stones. He said he occasionally experimented with his own jewelry designs, although he did not copy vintage styles. For example, in one ring, he layered a portrait-cut diamond atop a pink diamond measuring one-tenth of a carat, giving the combination a kind of mysterious attraction. “The possibilities with these stones are endless,” Mr. Finkelstein said.

Once she could get her hands on them, Kelty Pelechytik, of Edmonton, Alberta, explored the possibilities of portrait-cut diamonds, making them a significant part of her brand’s output.

She said she waited a year for a gem dealer to ship her first parcel of portrait-cut white diamonds, and once she received them, in 2018, she incorporated them into all manner of pieces, including hoops and eternity bands.

Her contemporary interpretations of lover’s eyes, a jewelry genre dating from the late 18th century that featured miniature images of a single eye or set of lips, have proven particularly popular over the last year, she said. Clients submit a photograph of an eye or lips, then Robyn Rich, an artist in Frankston, Australia, renders it in enamel on a gold surface, and Ms. Pelechytik sets it under a clear portrait diamond.

The jewels, which range from about $4,300 to $15,000, are often romantic gifts or keepsakes, but Ms. Pelechytik said she also had made pieces to honor deceased parents and “many people commission them of their children and pets as well.”

Some designers have been longtime proponents of the portrait cut. Eva Zuckerman said she had used portrait-cut diamonds “since Day 1” of her decade-old fine jewelry brand Eva Fehren, a time when the style was so rare that she “collected and coveted them for years and years.”

While the New York-based designer views the diamonds as “extremely beautiful with a special luster,” she said the fact that they defied expectations was part of their appeal. “I think there’s something quite irreverent about using a diamond that isn’t cut in a way to maximize its sparkle,” Ms. Zuckerman said. “I think it’s a little rebellious.”

She initially used portrait-cut gems for small hoops that hug the ear, pendants and stacking bands, but has expanded their use to more extravagant designs, like a pair of shoulder-grazing stiletto earrings. “We’ve gotten gutsier with scale,” Ms. Zuckerman said. “Now I feel more empowered to take risks because I’ve seen that there is an appetite for them.”

In 2019, Ms. Zuckerman even chose a hexagonal portrait-cut diamond, shaped to her specifications, for her own engagement ring. “I love them for so many reasons, but part of it was the wearability,” she said. “I use my hands all the time, and portrait-cut stones are low profile and understated.”

She doesn’t mind that the cut ensures that the gem is not easily identifiable. “Most people might not recognize it as a diamond,” Ms. Zuckerman said. “But if you know, you know.”