This gem is where the actual soul of a 'magical girl' resides. 'Killing' her physical body has no actual effect of killing the girl herself; the body is just a surrogate that can be re-produced by this gem, given enough time, biological matter and ambient energy (Hellerium works perfectly well, although electric current and sunlight will also do, only orders of magnitude slower); this works in a similar fashion as the fabled Regeneration Chambers. It is possible to influence this process (and the kind of body produced) by providing certain substances to the gem - we'll need the understanding of Genetics and Surgery Units to do that. I guess this makes a 'magical girl' some sort of a bio-cyborg, quite possibly constructed by the Purple Bloom herself.