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.

Fun fact: while Hawking radiation will eventually evaporate away almost all of a black hole's mass, the black hole will eventually become small enough that physicists think the system would stabilize (because it would have so little mass that it would actually have to reduce entropy in the system in order to evaporate any further). It would then just wander the universe, interacting with gravity in a tiny way, but being utterly invisible to any other means of detection we have. Add to that the fact that there were likely a huge number of black holes in the early universe, which was long enough ago for sufficiently-small black holes to have evaporated to this stable state, and you come up with a plausible explanation...for dark matter.