HONG KONG - The Argyle Pink Diamond Tender, showcasing the finest coloured diamonds from Rio Tinto Ltd's mine in northwest Australia, opens its doors to about 100 carefully selected gem enthusiasts in Hong Kong this week.

Jewellery makers, connoisseurs, collectors and investors - by invitation only - will view the 64 diamonds of this year's tender, which have a combined weight of 54.99 carats.

Besides 58 gems in various hues of pink, the catalogue features three blue and three exceedingly rare "Fancy Red" diamonds, one the biggest ever to be displayed at the tender.

Rio Tinto believes the United States will remain its key market for diamonds but China is growing fast as the increasing affluence of the country's middle-class drives up demand.

"The Chinese market is about five percent. We do expect, through the course of 2020, for that to be over 20 percent," said Alan Davies, chief executive of Rio Tinto's diamonds and minerals division. "So there's good growth there."

Tiffany & Co said last month that demand for gems in China had lifted its global sales in the latest quarter, leading the luxury jeweller to raise its profit forecast for the year.

Pink diamonds comprise just 0.01 percent of the Argyle mine's total production but they make up more than 90 percent of the pink diamonds on the global market.

The rarity of the gems pushes up the price to about 20 times that of white diamonds. Argyle's diamonds in the category sell for more than $1 million per carat.

As far as the Fancy Reds go, the Argyle mine has only ever produced nine of the rare gems, including the three at this year's tender.

The price?

"It's extraordinarily hard to put a price on (Fancy Reds) and, frankly, the imagination is the limit," said Davies.

Chinese demand for diamonds reaches beyond the traditional solitaire engagement ring. It is increasingly common for customers to buy diamond-studded fashion jewellery for themselves, rather than just as gifts for others, Davies said.

To develop this aspect of the Chinese market, Rio Tinto has partnered with Chow Tai Fook Jewellery Group Ltd, which has made Rio Tinto's jewellery and gems available in 1,000 of its stores across the greater China region.

The flagship Australian Colors collection, which launched at the World Expo in Shanghai in 2010, has generated double-digit year-on-year growth for Rio Tinto.

Hong Kong is the last leg of the tender after viewings in Sydney, Perth, New York and Tokyo.