I don't know but looks as some pumice stone to me
... it's sintetic or real light stone?
My understanding is that it is a fired ceramic media.
Probably similar to 'sintered glass' or 'ceramic' bio-media.
The pore structure of this media just appears to be superior compared to other types of ceramic media.
Maybe comparing open pore foam like Poret foam to regular sponge foams is a similar comparison.