A diamond may be forever, but Chinese demand for the precious gemstones isn’t always so loyal.

China’s appetite for diamonds has waned recently alongside the volatility in the country’s stock market, Jean-Marc Leiberherr, managing director of Rio Tinto’s diamond division, said in an interview this week.

“We see that people are postponing their nonessential purchases and avoid buying overly conspicuous goods,” he said.

According to the Australia-based mining company, while China is still the second-largest diamond market in the world after the U.S., the continuing global economic slump and China’s own decelerated growth mean the brighter days of double-digit annual percentage growth in diamond sales seen between 2010 and 2014 is likely a thing of the past.

Other industry powerhouses such as De Beers, owned by Anglo American PLC, and Russia’s Alrosa have also offered dim views of the diamond market recently. Last month, both companies said diamond prices fell 6% to 8% in the first half this year. Mr. Lieberherr estimated global demand will contract by 5% to 7% in 2015.

Thanks to the frosty consumer demand, Chinese retailers such as Hong Kong-listedChow Tai Fook Jewellery Group Ltd., the world’s biggest jewelry chain, are stuck with large inventories that could take 12 months or more to clear, Mr. Leiberherr said.

“What the Chinese market is experiencing now,” Mr. Leiberherr said, “is a clog in the supply chain.”

Still, Rio Tinto remains optimistic about the longer-term prospects for the Chinese diamond market. Mr. Leiberherr said the company expects the industry to grow by 8% to 9% a year for the decade beginning in 2017, mostly driven by Chinese demand.

China’s taste for diamonds is evolving thanks to the country’s growing middle class, Mr. Leiberherr said.

Between 2001 and 2011, the share of Chinese who have middle incomes jumped to 18% from 3%, according to a recent Pew Research Center report.

“We see China’s demand for high-end diamonds is largely stable because diamonds are seen as a status symbol for the rich, but we also see a growing demand for the low-end diamonds used for fashion jewelries,” Mr. Leiberherr said. “Middle-class Chinese are not buying the high-quality diamonds at moment, but at least they are getting into the diamond category. And that to us is always a good sign.”

But a wider consumer base and stronger purchasing power aren’t the only factors driving Mr. Leiberherr’s confidence in the Chinese market.

“What better way to express love than through diamonds? The young people are no longer interested in lifelong commitments, but they are interested in love,“ he said. ”And love is going to be forever, right?”