Global diamond prices keep their slow declines, according to the IDEX Online Polished Diamond Index. The Index closed at 1039.9 on Monday, setting below 140 for the first time since June 18, 2011.

Diamond prices started turning downwards in mid-August, after the close of the IIJS show in Mumbai. Loose diamond sales at the show were below expectations, against a backdrop of falling stock markets around the world.

From their peak in August 17, the index declined 4.6 percent, declining 1.5 percent in the last 18 days.

Fancy Cuts Outperform Rounds

Large Rounds suffered the sharpest price declines in the last 30 days. The average price of 4-carat Rounds dropped 50 percent, 5-carat Rounds lost 40.8 percent of their asking price and 3-carat Rounds 33.6 percent.

Bucking the trend, the price of a some diamond categories keep rising, most notably 5.00 carat Emerald cut (D-J / IF-VS1) goods, rising 2.7 percent since August.

While suffering from declines since August, month-over-month, the average price of 1-carat Princess cuts (D-I / VVS1-SI2) soared by 32.5 percent, 1.50 carats Princess goods (E-I / VVS2-SI2) jumped 30.4 percent and 1-carat Emerald goods (E-G / VVS2-SI1) increased 29.3 percent.

The IDEX Online Polished Price Index is calculated in real-time from actual asking prices of more than 400,000 diamonds from the global diamond industry.