A giant diamond nicknamed "The Rock" went on display for the first time in Dubai on Friday ahead of an auction where it is expected to fetch up to $30 million.

The 228.31-carat pear-shaped gem, which was mined and polished in South Africa more than 20 years ago, is the largest white diamond ever to come to auction, said auction house Christie's.

After making its debut at Christie's Dubai, where it will be displayed from March 26-29, The Rock will travel to Taipei, New York and Geneva, where it will be auctioned on May 11.

"The Middle East has always had such a great appreciation for important jewels and gemstones," Rahul Kadakia, international head of jewellery at Christie's, told AFP.

"We thought it would be nice for us to launch the diamond in an area where there are so many great collectors for important gems of this nature."

The diamond is the "largest existing D-Z colour pear-shaped diamond ever graded" by the Gemological Institute of America, Christie's said in a statement, adding it is "G colour, VS1 clarity".

The previous auction record for the largest colourless diamond was a 163.41 carat sparkler which sold in November 2017 for $33.7 million, Christie's said.

Earlier this year, Sotheby's in London sold "The Enigma" -- the largest cut diamond ever to come to auction, at 555.55 carats -- for $4.3 million.