118-carat diamond could fetch $38 million

The oval white diamond that was auctioned Monday.
The oval white diamond that was auctioned Monday.

Diamonds never go out of style, but they are becoming much more expensive.

Sotheby's plans to auction a giant D-color diamond next month that could well become the most expensive rock ever sold at auction.

The oval-shaped stone is 118.28 carats—about the size of an egg. It's flawless (type IIA) and is estimated to sell for between $28 million and $38 million when it goes under the hammer on Oct. 7.

(Read more: Want to invest in diamonds? Good luck with prices)

The stone was discovered in 2011 from the deep mines in South Africa and was 299 carats before it was cut.

Sotheby's will be selling the stone in Hong Kong—a sign of just how much of the big-jewel business has shifted East. Hard assets of all types have surged in value in recent years, as the wealthy look for safe stores of value. During this time, Asian buyers have become especially avid diamond buyers.

(Read more: The world's richest diamond owners)

Mahyar Makhzani, the London- and Geneva-based managing director of the Sciens Colored Diamond Fund, estimated that more than 80 percent of large-diamond sales over the past three years have gone to Asian buyers. Many of those buyers are newly rich Chinese, Singaporean and Indonesian millionaires and billionaires looking for a safe place to put their money.

"These are people with a lot of liquidity and there are not many places to park it that also have a chance of holding their value," Makhzani said.

The giant diamond could break the current auction price record for a white diamond: the $26.74 million paid for the "Winston Legacy" this spring and sold by Christie's.

Sotheby's is selling a blue diamond that could set an all-time record for price per carat. The stone is called the Premier Blue and it's a 7.59-carat round cut, flawless fancy vivid blue diamond. It's expected to sell for more than $19 million, or well over $2 million per carat. The record paid per carat for a blue diamond at auction is $1.68 million, set in 2011.

(Read more: Huge Diamond Sold for $26.7 Million)

Round-cut blue diamonds are extremely rare and valuable, since the process of creating a round cut leaves more waste than other cuts.

"It's a terrific diamond," Makhzani said of the Premier Blue. "And very attractive to investors."

—By CNBC's Robert Frank. Follow him on Twitter @robtfrank.