"Blue diamonds are one of the most coveted colors of all diamonds," said Shor, senior industry analyst at the Gemological Institute of America. A colored diamond pops up about once among every 10,000 diamonds mined. "And a really prime blue diamond is 1 in 10,000 of those," he said, "so you're talking about maybe three or four out of all the millions that are mined each year" that are more than a carat. Thus, the blue diamond is an "exceptional rarity."
What's the very rarest? The red diamond. There may be 20 known red diamonds in existence, Shor noted.
Petra Diamonds has owned the Cullinan mine since 2008. But Cullinan has been in operation for nearly 110 years, Shor said. The famous Taylor-Burton diamond was found there. And in 1905, the largest-ever diamond -- the Cullinan diamond -- was mined. That behemoth was 3,106 carats. It was cut into nine stones, Shor said.
"The largest one, which was 530 carats, was mounted in Queen Elizabeth's scepter," on display in London at the crown jewels exhibit.
In a statement Tuesday, Petra Diamonds said the newly discovered gem "is a vivid blue with extraordinary tone and clarity, and could yield a polished stone of great value."
Experts will likely spend some time studying this stone, according to Shor.
"It's not like years ago where you had a guy with a loop hunched over a stool," he noted. "Today they do it by computer modeling, and you can superimpose diamond shapes.
"The first thing they'll look for is inclusions," or flaws in the diamond, "and also cracks or fissures that might make the stone unstable .... Then you try to cut around those. And you need a shape that's appealing."
The diamond will be cut to maximize "the body color," he said. "Typically the yield is about 30% to 40%" of the stone, plus some smaller "satellite stones."
Another blue diamond, weighing 25.5 carats, found at Cullinan last year sold for $16.9 million, so the new blue beauty stands to grab an even higher price.