The "Blue Moon Diamond," a 12-carat gem described as one of the rarest stones in the world, was unveiled by the Los Angeles County's Natural History Museum on Friday.
Deep blue and intensely radiant, it was found in South Africa's Cullinan mine northeast of Pretoria, where the biggest ever rough diamond was unearthed a century ago.
"Fancy vivid blue diamonds are extremely rare and the Blue Moon is no exception," said Suzette Gomes, chief executive of Cora International, a leading diamond supplier from whom the gem is on loan.
"It is a historic stone that is one of the rarest gems with this color and in this size to be found in recent history," she added.
Only a few bigger blue diamonds exist, including the "Heart of Eternity," valued at $16 million, and the 45-carat "Hope" worth an estimated $350 million despite having a reputation as being cursed.
The value of "Blue Moon," which took six months to be cut from a 30-carat rough diamond, is not yet known but Gomes noted that Cora bought the rough diamond for $26 million.
The Cullinan mine is where a 3,106-carat diamond was discovered in 1905. It was presented to the British monarch King Edward VII and then cut up, with two of the principal resulting gems forming part of the British crown jewels.
These are the "First Star of Africa" (530 carats) and the "Second Star of Africa" of 317 carats, now held in the Tower of London.