Blue diamonds are ultrarare, as are stones weighing more than 100 cts.—making this, as Petra puts it, a “truly significant find.” “It’s virtually unheard of to have a blue of over 100 carats,” says spokesperson Cathy Malins.
However, it’s not clear whether this will stand as the largest piece of blue rough ever found. One reference source says that a 620.14 ct. light blue stone was recovered in South Africa in 1984. Beyond that, there is no official record of the size of the rough was that produced the world’s most famous blue, the 45 ct. Hope, but it is estimated at 112 cts.
A statement from Petra did not comment on the color or clarity of the stone, saying it will require “further analysis in order to assess its potential value.” But an analyst told Reuters he believes it could set a new benchmark for the highest recorded price of a piece of rough when it’s sold later this year—besting Petra’s previous champ: the Feb. 2010 sale of a 507 ct. white stone for $35.3 million.
Thomas Gelb, educational director of the Natural Color Diamond Association, believes the stone has the potential to become a sizable piece of polished—possibly the largest blue stone ever.
“Given that it is from the Cullinan mine it is very likely a type IIb boron-rich diamond,” he says. “Type IIb diamonds are generally less likely to cleave or fracture during cutting and very frequently have few internal inclusions.”
The find occurs as the market for fancy colored gems continues to sizzle. Last month the 13.22 ct. fancy-vivid Winston Blue fetched $23.8 million at Christie’s, setting a new record for a blue diamond of $1.8 million a carat. A Petra statement noted: “This sets apart blue diamonds as one of the most highly concentrated forms of wealth known to man.”
The Cullinan mine—which Petra purchased from De Beers in 2008—has produced a number of significant blue stones, including a 39.9 ct. diamond, which sold for $8.8 million ($220,551 per ct.) in 2008; a 26.6 ct. diamond, which was eventually polished into the 7 ct. fancy-vivid blue Star of Josephine; and a 25.5 ct. diamond, which sold for $16.9 million ($663,144 per ct.) in 2013.