(IDEX Online) - A white diamond described as the largest ever to appear at auction could sell for $30m.The 228.31-ct pear-shaped gem will lead Christie's Magnificent Jewels sale in Geneva.The G/VS1 stone, named The Rock (pictured), was mined and polished in South Africa over 20 years ago. It has an estimate of $20m to $30m.The previous auction record for the largest white diamond was a 163.41-ct diamond which sold at Christie's Geneva in November 2017 for $33,701,000. The D/IF Type IIa rectangular cut-cornered diamond was set in a necklace with emeralds.Rahul Kadakia, Christie's international head of jewellery, said: "The Rock will join the very best of legendary gemstones which have passed through Christie's global salerooms since 1766."He told Bloomberg News that Christie's had originally brokered the private sale of the gem to a collector nearly two decades ago. The buyer recently approached them to sell it.The sale takes place at the Four Seasons Hotel des Bergues Geneva on 11 May.