Diamond miners at Petra Diamonds have recovered a significantly rare blue diamond at the famous Cullinan mine in South Africa. This spectacular rough blue diamond weighs a huge 25.5 carats and apparently has been classed as ‘top colour’ for rough blue diamonds. What makes this diamond more appealing is its origins as the Cullinan mine has held many of the world’s largest and rarest diamond. Petra Diamonds are a leading independent diamond mining group that supply rough diamonds to the international market. The diamond group have previously mined a rough blue diamond like this weighing 26.6 carats, which was polished into a 7 carats flawless fancy vivid blue diamond and sold at auction for $9.49 million back in 2009. This rare 7 carat diamond was named The Star of Josephine.