Subject: What makes a rock spongy?

I have a question about the identity of a rock.  

Description:

A deposit of globular, mushroom shaped fulgerites(?) on the shore of Lake Michigan.  Each specimen 
has a hard 
black coating (0.5 mm) on the outside, but can easily be broken open by hitting each with a heavy 
object or applying other mechanical stress.  Sand on the inside appears to be fused together with a 
hard / spongy consistency. The rock is soft enough that you can push a needle into it.

A cross section reveals ringlike structures of blackened sand.  Microscopically, the sand grains appear 
to have fibers surrounding each grain.  Macroscopically they fook fuzzy. Bits of the rock produce 
bubbles in 1 N HCl, but do not fall 
apart after prolonged exposure to acid, and take on a more spongy consistency.


What the heck are these things? Some kind of fungal colony? Maybe a strange variant of a fulgurite?



Please let me know what you think when you get a chance.

Re: What makes a rock spongy?