MSC Cruises introduces the Blue Diamond tier for its most loyal guests, offering exclusive perks and an extended membership.

"The diamonds can range from clear to very deep blue," Martoia said. "The more boron, the deeper the blue." She added that it's impossible to predict the exact color a memorial diamond will take on.

A very rare blue diamond sold for over $40m (£32m) at a Christie's auction in Geneva, news agencies said. The 17.61 carat, pear-shaped Bleu Royal diamond, set in a ring, fetched $43.8 million ...

It was the disappearance of one piece in particular that caused consternation, however: a rare 50-carat blue diamond the size of an egg. Only about 1 in 10,000 diamonds has a distinct body colour ...