According to this article about diamond prices plummeting this year:
Falling marriage rates as well as growing popularity for gold and lab-grown gems all drove down Chinese demand for diamonds
...
“There’s no doubt that there are some challenges in the diamond industry, but they’re not challenges that can’t be addressed,” said [the] co-founder of specialist diamond advisory firm Gemdax.
He noted diamonds are discretionary products and it’s a case of “creating the want” for it, as with the case for other luxury segments like high-end watches and bags.
“The industry has not done large-scale category marketing for almost 20 years. And we’re seeing the aftermath of that”