I have a theory…

The diamonds knew about different gems fusing, and how their individual (gems’) differences made the fusion ultimately stronger.

They feared that if enough weaker/different gems fused, that they’d be overpowered if it came down to the multitude of different gems fusing against their potential 4 way fusion. They might have said that different types of gems fusing was an abomination as a form of control.

I also think that’s why they created the cluster. A number of broken (no pun intended) psyches to eventually form and destroy the planet containing the only gem to figure it out.

Rose Quartz found out, and that’s why she said Garnet was “the answer” when she asked Rose what she was upon their first meeting.

And, other theories suggest that Rose, Garnet, Amethyst, and Pearl fused in order to shatter Pink Diamond. 5 gems potentially fused to shatter just one diamond. How terrified would they be of 50, or 100 gems?

This might be why they buried The Cluster in the core. For a bit of irony. Yes, enough individual gems might shatter them, but shattered individuals, forced to form, would be the Earth’s demise.