Even in the world of rare stones, Foxfire is a freak.

It was buried in a place where big gem-quality diamonds aren’t supposed to exist. A Rio Tinto Group ore processor was configured to discard it. And what saved the diamond’s 187.7 carats from being pulverized was a fluke: Its unusual, elongated shape allowed it to slip sideways through a filtering screen.

“It really is a miracle that it was found,’’ said Alan Davies, chief executive officer of diamonds and minerals for Rio Tinto, the operator of Canada’s Diavik mine, Foxfire’s former home. “It’s a rare find, a really rare find.”

That’s the company’s marketing line as it shows Foxfire to prospective suitors on a worldwide tour and promotes it as the largest gem-quality diamond ever found in North America. Luckily for Rio Tinto, rare diamonds are hot, much hotter than bog-standard rough stones. Sales of those fell 18 percent last year, while their uncommon cousins rack up records. Lucara Diamond Corp. just sold an 813-carat jewel named the Constellation for $63 million, making it the most expensive of its kind—$77,649 a carat. Next month, Sotheby’s will offer one that could fetch more, the Lesedi la Rona, which at 1,109 carats is the size of a tennis ball.

David Fuller's view

I remain an unabashed bling fan, although fortunately never wishing to wear it myself. 

Diamond prices have slipped in the disinflationary environment, although not the best examples of rare and exceptionally large gems.  Looking ahead, I think diamonds and other quality examples of precious stones will appreciate as investors reduce some of their exposure to equities and particularly bond markets, which are priced for no inflation and no economic recovery.