Nov. 26 (Bloomberg) -- For today's brides, diamonds are still forever; they're just not always round anymore.

In the $6 billion market for diamond engagement rings, square-cut designs, notably worn by actresses Ashley Judd, Brooke Shields and Reese Witherspoon, are booming in popularity. Witherspoon sported an Asscher square-cut stone, distinguished by long, slender facets and beveled corners. Judd and Shields flash a cushion style, with curved sides, rounded corners and triangular facets.

While round diamonds are still the most sought-after shape, representing 47 percent of diamond engagement rings, that's down from 55 percent a decade ago, the New York-based Diamond Information Center says. Square cuts have grown to 30 percent from 9 percent in 1999.

``I had seen so many round diamonds, on friends and family, so I just wanted something a little more unique,'' said Lorna Blaire, who recently decided on a cushion cut for her ring, a 2- carat gem set on a diamond and platinum band. ``It looked pretty on my hand. It pops out.''

The months of October through December accounted for 37 percent of U.S. sales of diamond engagement ring in 2006, when more than 80 percent of brides received a diamond engagement ring.

The holidays are ``a time for gathering the family, and it's a great time to show your ring,'' said Hamida Belkadi, chief operating officer of the U.S. arm of London-based De Beers Diamond Jewellers Inc.

Record Prices

Blaire, a 31-year-old kindergarten teacher who lives in Great Neck, New York, plans to brandish her ring when she, her fiance, relatives and friends spend a week together in Aruba between Christmas and New Year's. She and Gregg Fishbein, 30, a pharmaceutical salesman in New York City, got engaged in May and are marrying in July. Blaire said she didn't know the price of her ring.

Last year, Americans paid a record average price of $3,200 for the 1.9 million diamond engagement rings they bought.

The diamond solitaire engagement ring became popular after World War II, helped by De Beers's ``A Diamond Is Forever'' marketing campaign.

U.S. demographics point to growing purchases. Marriages will peak at 3 million annually in 2016, up from 2.3 million during the 1990s, said Ken Gassman, president of the Jewelry Industry Research Institute in Glen Allen, Virginia.

At jeweler Harry Winston, where engagement rings start at $18,000, the trend toward square cuts has generated sales of its emerald cuts set with tapered baguettes, as well as its cushion cut in a micro-pave -- diamond-studded -- setting, spokesman Carson Glover said.

Asscher Cut

The cushion cut dates back to the 18th century. The original versions, meant to be worn in candlelight, glow softly. The Asscher was designed in 1902 by the diamond-cutting Asschers of Amsterdam.

Cutters now use modern technology to get a more brilliant shine out of the older shapes. Jewelers began offering such cuts when the supply of vintage jewelry couldn't meet the demand as those styles became trendy, Penny Proddow, co-author of ``With This Ring,'' said in an interview.

Tracy Lantz, a spokeswoman for the Diamond Information Center, reflects the trend toward square shapes.

``I received a round diamond from my first husband in 1999, which is what I wanted at the time,'' Lantz said. ``I wanted an Asscher when I got engaged to my now-second husband last year.''

The princess cut was created by London jewelry designer Arpad Nagy in 1961. It provides the same kind of sparkle as a round brilliant cut, yet the flat square shape makes the same carat weight appear larger, Lantz said.

New Tiffany Designs

Seeking to capture the new demand, New York-based Tiffany & Co. introduced a square ring, the Novo, in November 2006.

The Novo is based on the 128.54-carat Tiffany diamond on permanent display in the Fifth Avenue store in Manhattan. The Novo is a cushion-cut solitaire, set on a diamond and platinum band. A 1.01 carat version sells for $12,700.

It follows the Legacy vintage-style ring introduced in 2004 by Tiffany, the world's second-largest luxury jewelry retailer after Cie. Financiere Richemont SA, of Geneva. In the Legacy, diamonds surround a cushion-cut stone.

In 1999, Tiffany unveiled the patented Lucida square diamond cut, which has wide corners and as many as 50 brilliant facets. It is set on a wide, plain band.

The round-brilliant diamond was invented by Belgian-born cutter Marcel Tolkowsky in 1919. Its 58 triangular and kite- shaped facets allow light beams to bounce back through the crown.

Gassman at the jewelry research institute said men who are remarrying this season may be facing an even larger shopping task.

``Sometimes a second wife says, `I better have a bigger diamond than the first wife,''' he said.

To contact the reporter on this story: Cotten Timberlake in Washington at ctimberlake@bloomberg.net.

Last Updated: November 26, 2007 00:07 EST