The growth in demand isn't keeping up with population growth because young people are not getting married, don't have enough money, read on the internet who mines the diamonds. Screw diamonds and the idiots who are upset they are getting cheaper.
People will call it "virtue signaling" or whatever, but ultimately a lot of diamond mines around the world are run on blood (war, slavery, abuse, dangerous work conditions, etc) and they want no part of that, and are happy to let others know they made that choices.
So the diamond industry is getting squeezed from three sides. Customers too poor, inexpensive lab grown out of China, and a customer base who would rather not be associated with this: https://time.com/blood-diamonds/