this gemstone stands out as part of the elusive 0.02 percent of diamonds with natural color. Its rich azure hue is the result of a unique chemical process deep within the Earth's crust.

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 ...

Hope Diamond at the Smithsonian Natural History Museum ...

"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.

The scarcity and value of blue diamonds is in part down to the fact so few places in the world mine for them. The stones get their colour when the chemical boron is present during formation.