You know how gems are made?
PRESSURE.

So of course they are using it for the cluster gem fusion thing.
BUT

MALACHITE.

Malachite is at the bottom of the OCEAN.
And you know what’s at the bottom of the ocean? 
PRESSURE.  (15,750 psi to be precise)

What if that affects them over time? And that’s why Lapis is keeping them at the bottom of the ocean?