Re: What makes a rock spongy?

The real answer is that I do not know.  I've consulted with a couple 
geologists and they also do not have a definitive answer without being 
able to examine the rock.  But here are a couple ideas.

We assume that the sand contained iron oxides that became fused during 
the lightning heating event.  Fulgurites are funny and unpredictable, and 
it is not unusual for the insides of these structures to be non-uniformly 
fused such that there could be loose sand remaining.  The black crust is 
possibly magnetite.  We speculated that perhaps the small fibers you see 
are hydrated iron oxide growths such a hematite.  There are cases where 
the lightning hit sand containing roots or other organic matter; this can 
leave unique imprints in the resulting fulgurite.

Again, without seeing the rock, it is really hard to say.  Probably worth 
taking this to the geology department at a local college or locating a 
local rockhounding club.