A 101.27 carat diamond the size of a squash ball will be sold by auction house Christie's in Hong Kong next month and is likely to fetch up to $7.5 million.The gem is the largest colourless diamond to show on the auction market in 18 years and one of only three colourless diamonds of over 100-carats to have ever been auctioned, Christie's said.

The gem, which is internally faultless but rated VVS1 (very very a little imperfect 1st degree) for simplicity is expected to fetch upwards of $6 million -- a relatively low per-carat price.A 39.34-carat "D" colour, flawless diamond sold for $6.87 million at a Christie's New York sale earlier this month. While rare sophisticated diamonds achieved all-time highs per carat at sales in Geneva late last year, a much-hyped 72.22 carat "D" flawless white diamond failed to sell at a Sotheby's Hong Kong auction earlier this month, suggesting demand for top stones may be waning due to a weakening world economy.

The 101-carat "shield-shape" stone was cut from a 460-carat hunk of rough precious stone and mounted on a tiara, rather than a choker or pendant given its extraordinary thickness.

"With Asia becoming such a vibrant market, it is fitting that this rarest gem should be offered ... to the growing number of collectors in the area," said Francois Curiel, Christie's chairman of jewellery, in a statement.

Naming rights for the gem will be granted to the buyer, who Christie's said might come from Russia or the Middle East.

Other items to feature in Christie's May 28 sale include a coloured diamond ring set with a fancy green diamond estimated to be worth $3 million, as well as a rare emerald and diamond ring set with a 14.31 carat emerald that could fetch $800,000.