Cushion-cut diamond split-shank ring set in platinum, Tiffany & Co., tiffanyandco.com, 800-526-0649.

This modern cut, created in the 1960s, boasts an elliptical shape that appears larger than a round stone of the same carat weight. Ring, Harry Winston, harrywinston.com.

In 1919, the mathematician Marcel Tolkowsky calculated the measurements for the most brilliant diamond -- the round. Micropave setting with heart prongs, Ritani, ritani.com.

Talk about sophisticated -- this striking shape dates back to ancient Egypt; Cleopatra was said to have favored it. Ring, De Beers, debeers.com.

Initially introduced during the Renaissance, the pear was re-popularized by Elizabeth Taylor, who famously wore a 69.42-carat pendant -- a gift from beau Richard Burton. Ring, Tiffany & Co., tiffanyandco.com, 800-526-0649.

This shape, with its sharp corners, first appeared in the 1960s. It's a square version of a round brilliant cut -- which explains why it's also known as the square-modified brilliant. Ring, Kwiat, kwiat.com, 212-725-7777.

The deeply trimmed corners and parallel facets of this octagonal shape produce far more fire than most cuts. "Deco" ring, Royal Asscher, royalasscher.com.

Emerald-cut diamond ring, Van Cleef & Arpels, vancleef-arpels.com.

Handcrafted princess-cut diamond ring, Kirk Kara, kirkkara.com.

Princess-cut diamond ring, Diana, diana.com.

Platinum-and-diamond setting for emerald-cut center stone, Tacori, tacori.com.

"Cocktail Fizz" white-gold and baguette-diamond ring, De Beers, debeers.com.

Radiant-cut diamond in platinum setting, Varna, varna.com.

Emerald-cut diamond ring with tapered baguette side stones in platinum setting, Harry Winston, harrywinston.com.

"Avant Twist" Royal Asscher-cut diamond set diagonally in 18-karat white gold, Royal Asscher, royalasscher.com.

Emerald-cut diamond in platinum setting, Bulgari, bulgari.com.

Platinum and princess-cut diamond semi-mount for princess-cut center stone, Peter Storm, peterstormjewelry.com.

Marquise-shaped diamond in a pave setting, OGI Ltd., ogi-ltd.com.

Pear- and marquise-shaped diamonds in a platinum setting, Ritani, ritani.com.

Pear-shaped diamond ring, Harry Winston, harrywinston.com.

Marquise-shaped diamond set horizontally, Kwiat, kwiat.com, 212-725-7777.

Crisscut cushion diamond center stone surrounded by round diamonds and set in platinum, Christopher Designs, christopherdesigns.com.

Vintage cushion-cut diamond engagement ring, circa 1890, Single Stone, singlestone.com.

"Juliet's Court Open Fleur de Lis" platinum and round-diamond ring, Katharine James, katharinejames.com.

"1895 Collection" platinum and round-diamond ring, Cartier, cartier.com.

"Felicity Ribbon" platinum and round-diamond ring, Hearts on Fire, heartsonfire.com.

Round diamond in platinum setting, Tiffany & Co., tiffanyandco.com, 800-526-0649.

Round diamond in 18-karat white-gold setting, Memoire Boutique Bridal Collection, memoire.com.

"Elle" platinum setting for round diamond, by Erica Courtney, from Michael C. Fina, michaelcfina.com.

Platinum "shared prong" semi-mount for round diamond, Gumuchian, gumuchian.com.

Oval diamond in platinum setting, Ivanka Trump, ivankatrumpcollection.com.

Oval diamond in platinum, J. Birnbach, jbirnbach.com.

"Charlotte" 18-karat-gold and oval-diamond ring, Penny Preville, pennypreville.com.

18-karat gold semi-mount for oval diamond, Chad Allison, chadallison.com.

18-karat gold semi-mount for round diamond, Michael M, michaelmcollection.com.

Baguette semi-mount for oval diamond, Tacori, tacori.com.