Macle. Maccle. Maacle. Mackle. Little wonder few people know what this particular type of diamond is; even the experts don’t agree on how to spell it. Over the many centuries and countries where macles have been used, the spellings differ, and that continues today, between companies and sometimes, even within them.

But they do agree on one thing: A macle is a form of rough diamond used to produce jewelry that is singular, striking and sophisticated.

“They are for an inherently sophisticated customer,” Sally Morrison, director of public relations for natural diamonds in the De Beers Group, said by telephone from New York. “It’s a quiet, understated luxury. People may not know what they are, but you know it.”

And recently, more people know them, too, she said.

“We’re seeing more design using rough diamonds in their natural state,” Ms. Morrison said. “And I’ve seen more use in the consumer market of rough diamonds in their natural state. They have a glow and shine, but they are not brilliant in the way a cut and polished stone would be. They’re part of an overall trend to celebrate things as the earth made them rather than after a lot of human intervention.”

Macles have been around forever. “They were probably first used in jewelry in India about 2,500 years ago when diamonds were first discovered in Golconda,” Andrew Coxon, president of the De Beers Institute of Diamonds in the St. James’s quarter of London, said in an email. Today, he said, macles “are found in every mine production from all over the world. They can be attractive to discover, especially if they have tumbled for millions of years along a riverbed and acquired a natural shiny patina.”

What else sets macles apart? Greg Kwiat, the chief executive of Kwiat, a 115-year-old family-run brand based in New York, knows all about them. “The term maccle describes a specific type of rough diamond. It has a flat, triangular shape,” he explained in an email. “They occur naturally in the earth and are different from the more classic octahedral shape of rough diamonds.”

A macle is also a “twinned diamond crystal,” according to the Gemological Institute of America research librarian Judy Colbert in Carlsbad, Calif. It is composed of “two opposing parts, each with the same crystal shape. The two parts are oriented 60 degrees or 180 degrees from each other, so the macle looks like a flattened triangle.”

“Macles are a challenge to diamond cutters,” she said in an email. “Macles are usually so shallow that they can’t yield round brilliants without significant weight loss. For these reasons, macles are usually used for fancy shapes like pears, triangles, and hearts.”

Macles are a challenge to designers and jewelers as well. “Often a bezel setting is used as opposed to prongs, which are more commonly used for cut diamonds,” Elizabeth Gaines Zoutendyk, a G.I.A. design and manufacturing specialist, said in an email. “When prongs are used, they need to be bulkier and/or longer to ensure security. Both of these can mean an increase in the amount of metal required to create the setting.

“Instead of being able to use ready-made components that are created to standard sizes and often available in a pre-polished state (thus reducing labor time), a setting would be built by hand, to spec, for each individual stone (increasing labor time).”

That’s the process that was used to create the two wide cuffs that Nicole Kidman wore at the Academy Awards in 2007 to accessorize the long, lean red column of a Balenciaga evening dress. On her wrists were what looked like a mosaic of misty triangular stones, or crystals, or sea glass maybe; glowing rather than sparkling.

The bracelets were made by L’Wren Scott in collaboration with the jewelers at William Goldberg.

Eve Goldberg, co-owner and creative director of the company founded by her diamond-dealer father, William, who died in 2003, said he had been known for his expertise in working with “uniquely shaped stones.”

“We were presented with the large collection of maccles. We laid them all out on the table and just started playing around with them,” Ms. Goldberg said in an email. “They were flat on both sides and we are used to having a pointed culet on one side. It was fun to move them around in the diamond pad and see what design ideas might work. We collaborated on those fabulous cuffs because the stones just felt right that way. My dad always said that the diamonds talk to us and tell us what they want to be, and these diamonds told us they needed to be on an arm and so they were!”

The custom work that is often required with working with macles affects the price of a finished piece. “In general, uncut diamonds cost less than cut diamonds of similar quality and size,” Ms. Colbert said. “There are costs involved in the cutting process. Conversely, because uncut stones require a custom setting, this may result in an elevated price for the jewelry item.”

As with all diamonds, the price of the gemstone also varies with its quality. “When a beautiful large maccle is discovered in a fancy color it is preserved by De Beers to become a beautiful rough diamond jewelry piece of its own, for example as part of a high jewelry necklace,” Mr. Coxon said. “Such a maccle is as valuable as a polished diamond of the same size.”

In the end, Ms. Zoutendyk said, macle diamond jewelry can be “edgy, different, out of the ordinary, modern and streamlined.”

And something only those in the know, know.