(IDEX Online) - British miner Petra has recovered five extremely rare blue diamonds at Cullinan, in South Africa.The gems are Type IIb, of high-quality color and clarity, sized between 9.61 and 25.75 carats.Blue diamonds are reckoned to be the second rarest of all fancy color diamonds after reds. Petra says the last significant blue diamond it recovered at Cullinan was a year ago, so to find five (pictured) within a week is all the more remarkable.The gems have been assessed as individual stones and not originally part of one larger diamond.Petra says that given the challenges of COVID-19 it has yet to decide how it will sell the stones.The famous Cullinan mine was the source of the largest rough gem-quality diamond ever - the 3,106.75-carat Cullinan Diamond that became part of the British Crown Jewels. Owners Petra Diamonds has put itself up for sale as its debts mounts and revenue slumps.