My understanding is that the "rocks" are a product of the electroplasma system being exposed to air. Whether that is some sort of coolant that is meant to seal the EPS leak in that console or some sort of EPS byproduct reacting in air, I don't remember or have a head-canon for it.
All of that said, if I form this reply into one worthy of Daystrom, then I say it is an intended safety mechanism to protect the crew against catastrophic failure of the EPS conduits.
That's exactly the thought I had about it. (The sample return part.)
Perhaps also with a small containment field with its own power supply? Thus the smaller size would allow for more efficient use of such a field, whether by allowing it to be more secure while using the same power as a larger field or to sustain a smaller field for a longer time in cases of general power failure?