I think that happens in any black hole formation. At least that’s my understanding of how neutron stars are formed. The electrons get forced into the nucleus and turn the protons into neutrons. From there it’s quark gluon plasma then a black hole.

In any case, I have no idea how either a grain of rice or a mountain could be made to do such a thing.

Theoretically you could make a black hole with a single grain of rice. You just have to figure out how to crush it down enough.