Sponge wasn't the right word to use. yes, you're right, it's like a brittle rock and can be crushed between the fingers. That's what I should have said. Vermiculite holds moisture, but anytime I've read about perlite, the info stated the purpose was to hold moisture. More reading shows that it's used to retain water and air.
"In horticultural applications, perlite is used throughout the world as a component of soilless growing mixes where it provides aeration and optimum moisture retention for superior plant growth."
It really can't do both at the same time, which I guess is why vermiculite is added.