Dec. 4 (Bloomberg) -- The ultimate socialite accessory comes up for sale at Sotheby's New York on Dec. 6: a knuckle- skimming, bubblegum-pink diamond ring, topping 28 carats. It's sure to get noticed in Palm Beach or Moscow.

The rare rectangular rock, described by the GIA gem-trade laboratory as "fancy intense purple-pink," is the largest of its kind ever graded. In spite of the show-stopping size, the same diamond failed to sell in May 2005, when offered at Sotheby's Geneva with a $7 million to $8 million estimate.

"As gemstones go, this is like a $100 million painting," said Alan Bronstein, a New York colored-diamond private dealer.

This time around, the auction house convinced the anonymous seller to knock $500,000 off the estimate, bringing it down to $6.5 million on the low end.

"It's a totally different market than it was then," said Gary Schuler, director of Sotheby's jewelry department. "Now I expect it to surpass the estimate."

Schuler and other gem dealers say the market has improved.

"The diamond market is certainly stronger that it was a year ago," said Bronstein.

The auction record for a diamond -- for jewelry of any kind -- is held by Sotheby's New York for a 100.1-carat pear-shaped diamond sold in 1995 for $16.54 million.

Colors Now Wanted

Demand for colored diamonds is on the rise. "For the last three-quarters of a century, it's always been white diamonds," said jewelry dealer Kathryn Bonanno Patrizzi. "Fancy colored diamonds have now become really popular."

Desirable colors are pink, yellow, green, blue or red, the rarest and most expensive hue. Pink was popularized in 2002, when actor Ben Affleck gave Jennifer Lopez a six-carat pink- diamond engagement ring. When the relationship hit the rocks, Affleck reportedly returned the ring to jeweler Harry Winston for resale.

Sotheby's pink diamond is much larger and has a deeper, more saturated color. The ring is part of Sotheby's Magnificent Jewelry auction, which is estimated to total more than $25 million and includes a 1953-54 ruby-and-diamond brooch designed by Salvador Dali and various tiaras, diamond bracelets and art deco ear clips.

To contact the writer of this story: Lindsay Pollock at lindsaypollock@yahoo.com.