The diamond industry sucks don't get me wrong. But the real culpurists are the dumbfuck diamond buyers.
My friend is a diamond salesperson and told me a story about one of their customers. They were looking at different pieces and the customer kept asking about the purity of the diamonds in the piece. Whenever my friend said it's "SI," the customer would be visibly disappointed and would ask for "VS" or "VVS" which are purer. My friend then got annoyed a bit and told the customer that purity doesn't matter once you reach "SI" since the impurities are not really visible by the naked eye. He even showed the customer 2 pieces with one looking 10 times better than the other but has SI diamonds and the non-pretty piece has VS diamonds. He asked the customer to tell him which is which and the customer wrongly said the SI one was more pure. Even after he revealed his ruse and showed that purity doesn't matter much, the customer kept asking for more pure pieces as if nothing happened.
These "people" literallly are willingly being lied to, and they like it. If a diamond buyer saw a piece, told you they love it, told you they would buy it, then you told them it's a synthetic, they would be disgusted. It's bullshit from all sides and they deserve eachother.
I feel like the high-end buyer's don't care about how much they like their possessions, they care about how expensive their possessions sound when described to others. They couldn't tell that the SI was less pure, but they knew they couldn't describe it to their friends as the purest, so they didn't care.
Funny thing is that this comes from middle class people. There's still VVS and flawless diamonds which are significantly more expensive. But they can't afford one that looks any good. Given the choice they'd absolutely burn their money to buy those though.
There's a concept, 'conspicuous consumption,' that people will use products in such a way for their social power regardless of anything else. So getting higher quality diamonds, whatever imperceptible difference it has, is still worth it to be seen as affording the higher tier. One of the ways in which market economies poison the brain.