An internally flawless blue diamond, weighing 10.10 carats, was the star attraction at a recent press event in London.

The largest oval blue diamond ever to appear at auction is to be sold April 5 in Hong Kong.

"Blue diamonds are extremely rare, and when they are over 10 carats and flawless, they are quite special," Quek Chin Yeow, deputy chairman for Asia at Sotheby's auction house, said at the press event.

An employee holds a 10.10-carat internally flawles
An employee holds a 10.10-carat internally flawless blue diamond at Sotheby's auction house in London, March 15, 2016.

According to the website, most diamonds used as gemstones are colorless or have a very faint tint. However, colored diamonds, known as "fancies," can be extremely rare and valuable. The most valuable gemstones ever known have been fancy diamonds.

"This is a beautiful sky blue, baby blue shade of blue, and the shape is very rare,” said Chin Yeow. The gorgeous color, and the fact that it's an oval, "will make it a little different from everything else that's sold before and to come," he added.

The diamond, mined in South Africa, is one of a collection of 11 cut by luxury jewelers De Beers in London in 2000.

"De Beers cut these to celebrate the millennium here in London. So to have it offered here 16 years later now in Hong Kong for the private Asian collection, it is extremely wonderful," said Chin Yeow.

Only one of the diamonds has previously appeared on the market before, selling at Sotheby's in Hong Kong for more than $6 million in April 2010.

The diamond will be sold at Sotheby's Magnificent
The diamond will be sold at Sotheby's Magnificent Jewels and Jadeite auction in Hong Kong on April 5.

This rare gem will lead Sotheby's Hong Kong spring sale after being exhibited in London, Geneva, New York, Singapore and Taipei.

It is estimated to sell for $30 million to $35 million.