The largest collection of the world's rarest and most valuable diamonds was brought to Hong Kong last week under a veil of secrecy for a closed auction so exclusive that even the richest tai tais were denied entry. Only about a dozen of the city's most reputable diamantaires were offered the chance - against rival dealers in Tokyo, Perth, London, New York and Geneva - to see and bid for the 65 pink diamonds at Rio Tinto Diamond's Pink Tender. With the retail price of the cheapest in the collection estimated at over US$460,000 and the dearest at US$4 million, it had to be one of the most secretive diamond sales in the world. 'The diamonds at the tender start at 0.5 carats,' senior sales executive of Argyle Diamonds Gavin Pearce said. Joking that people who judge diamonds by their size are not ready for the exclusive market, he said: 'For those who think in big numbers, I like to tell them that a tender-quality red diamond is worth US$5 trillion per tonne.' Unlike white diamonds, where there is a formula to calculate cost, the pink diamond's worth is based on fine variations of colour that only experts can appreciate. A deep red diamond - the collection includes two - is worth 50 times more than its white counterpart. The source of the Pink Tender's stones is the Argyle mine in Western Australia, which produces 80 per cent of the world's pink diamonds.