De Beers sold $418m worth of diamonds at its tenth auction this year, 12% down from $476m at its ninth auction.
Parent Anglo American’s share price fell as much as 2.3% to a low of R207.51 after the announcement on Tuesday.
"While the trade in lower-value rough diamonds is experiencing a temporary slowdown as a result of the demonetisation programme in India, demand across the rest of the product mix continued to be healthy and overall sales remained in line with seasonal expectations," said De Beers CEO Bruce Cleaver.
"Pleasingly, sales were also significantly higher than those for the equivalent cycle in 2015."
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