A few days ago some exciting news came to our attention. Petra Diamonds Ltd reported a new find in the Cullinan Mine in South Africa. An extremely rare Blue Diamond weighing 20.08cts. 

This exceptional stone is only slightly smaller than the blue diamond recovered from the same mine in 2014, which weighed 29.6cts in the rough. The later was polished into a 12.03ct flawless cushion cut diamond, which was later sold for $48.5million in 2015- a record price per carat to date. The polished gem was named ‘Blue Moon’, an apt reference to the well-known saying ‘Once in a Blue moon’.
 
Natural Blue diamonds are Type IIb and get their colour from the presence of Boron in their crystal structure. Such Diamonds are extremely rare. The more common Type Ia and Ib diamonds have Nitrogen as an impurity, which can cause yellow colour. Boron has a smaller atomic number than Carbon which creates a vacancy in the crystal lattice, making Type IIb diamonds semiconductors of electricity. Type II diamonds can be blue, grey or colourless.
 
We are waiting in anticipation to find out the polished weight and shape of the new find, and of course the name it will be given – ‘once or twice in a blue moon’ perhaps?!