The rough diamond sales value amounted to US$520mln at April 16, down from US$563mln in the second sale of the year and US$586mln the same period a year ago.
Following the first three sales of the year, sales are more than US$100mln lower than the same time last year.
However, De Beers chief executive Bruce Cleaver was positive about the results.
“While the second quarter of the year is traditionally a seasonally slower period, we continued to see good rough diamond demand in the third sales cycle of 2018 as diamond businesses have focused on restocking following healthy consumer demand for diamond jewellery in the US and China,” he said.
RBC Capital Markets said the diamond market has been showing signs of stability this year and the industry appears to be working through inventories.
"Positive demand growth from the US, enhanced by the tax cuts, and a lower medium-term rough diamond supply outlook with both De Beers and Alrosa seeing 2019-20 production lower than previous estimates (c. 3mcts in aggregate); the diamond market is in an improving position, in our view," the analyst said.
"The results from the De Beers Third Sight are on the weak side however at $520m in sales down 11% on $586m from last year. For De Beers to reach our forecasts, they would need to average $528m per sight but this is traditionally first half weighted."
RBC said it continues to see De Beers on track with a stronger back half expected but the next couple of sales will "need to be monitored".