De Beers recorded the lowest sales for its seventh cycle since it started releasing its data in 2016 and allowed customers to delay buying smaller diamonds.
De Beers, the world’s largest source of rough diamonds by value, notched up sales of $505m in September compared with $507m in the same period a year earlier.
The number is provisional and may be higher once the actual sales are finalised.
In 2016, it recorded sales of $639m in its seventh of 10 annual sales events.
For the year to date, 2018 is so far the lowest of the three years, with combined sales of $3.93bn against the previous year’s $4bn and 2016’s sales of $4.195bn.
"De Beers provided sightholders with the opportunity to re-phase the allocation of some smaller, lower-value rough diamonds," CEO Bruce Cleaver said in a statement.
"Demand remains stable ahead of the September Hong Kong Jewellery & Gem Fair."
De Beers has about 80 handpicked clients called sightholders who are allocated parcels of diamonds sorted and aggregated in Gaborone. The 10 annual sales events are known as sights.
Rapaport, the world’s largest online diamond trader and source of information on diamonds, sounded a cautious note on the Hong Kong fair, which starts on September 12.
"Expectations for the show are mixed, with suppliers hoping to assess prospects for the Far East market amid US-China trade tensions and the weaker yuan. Hong Kong-based jewellers, having reported strong growth in the first six months, are more cautious about the second half of the year, and that may impact spending at the show," it said in a recent note to clients.