Is China to blame?

No that would be De Beers Group (the premier diamond cartel in the world) which has a long running history of purchasing all the valuable gems it can get its hands on and funding insane amounts on advertising both over and subliminal.

South African–British corporation that specializes in the diamond industry, including mining, exploitation, retail, inscription, grading, trading and industrial diamond manufacturing.[3] The company is active in open-pit, underground, large-scale alluvial and coastal mining. It operates in 35 countries with mining taking place in Botswana, Namibia, South Africa, and Canada. It also has an artisanal mining business, Gemfair, which operates in Sierra Leone.

While this cartel has historically controlled the market especially in luxury goods related diamonds, their investments and colonial efforts have kept them a major player in the world diamond market.

From its inception in 1888 until the start of the 21st century, De Beers controlled 80% to 85% of rough diamond distribution and was considered a monopoly.[4] By 2000, the company's control of the world diamond supply decreased to 63%.[5]

Diamond prices have been going down, I'm sure for a variety of material reasons that better educated comrades could comment on. That China may now be entering the industry, with its general focus on building a parallel power structure to incorporate and protect the global south AND its own VAST TRACTS OF LAND which it is still exploring for mineral exploitation, should be no surprise. Diamond resource control for industry is a necessary front to wage competition over.

Artificial diamonds are another factor, again a more knowledgeable comrade would be able to fill in the way this has shifted industrial diamond demand, the areas where you don't need BIG GEM to == BIG VALUE, think their application in cutting tools etc etc. But also artificial diamonds encroach on the traditional diamond cartel's territory. They've mounted a fairly weak counter-messaging effort to try and tell people that real diamonds are inherently more valuable (this is arguably "true" I suppose, a lot more labor, blood, lives and slavery went into those things. But do you want to carry that misery on your finger? Dangling from your ears?)

I haven't read the article. I don't need to. I don't know if they're digging them up, I don't know if they're cooking them in a lab, if they've bought out "illegal" mines on the African continent (getting pretty far out there, just acknowledging the limits of my knowledge, in any case Xi, Liberate the Diamond Slaves) both probably, but I do know one thing-

I know China is not "to blame" for a diamond price crash. This is what happens when cartels lose control. Assuming there even is a "crash," in any material sense, (its fucking diamonds for fuck sake) as with the housing crisis, the conditions for its catastrophic failure are to be blamed on the ruling capitalist class and the political systems that failed to reign them in before things reached point where the contradictions became untenable and something had to break for the house of cards to collapse.

tl;dr:

Diamonds have never been rare. Diamond production used to be a lot more labor intensive and its history and current state is still steeped in Actually Existing Slavery and Colonialism. The supply of diamonds has been rare due to overt cartel behavior fully sanctioned by the west since the late 1800s.

Now, the material conditions have changed and diamonds are entering the market from sources the cartel cannot control. This artificial supply limit is dropping and so the demand price is going lower. Ironically, with China doing the heavy lifting here, it's likely a situation in which this new supply of diamonds is extracted through far less barbarity, cost of human life and general misery.

Diamonds are really cool rocks. But they're not rare.

Obligatory Kanye West - Diamonds from Sierra Leone I believe the movie this featured in was a product of a major cultural shift in how the west understood the human cost of the diamond trade. Demand for luxury diamonds has dropped significantly. It was hallmark card bullshit but with shiny stones. FUCK THAT. I'd rather rock a platinum tooth than a diamond on my finger. Just sayin.