It's called sponge rock and lace rock. It should be in the range of thirty cents per pound according to a guy at Pebble Junction. Unfortunately, they won't order it.

When I was in Miami on a fishing trip, everyone used it as landscaping rock. I have been wanting to go back down there to try to find a place like Pebble Junction. I found these guys recently http://www.berniesrockandgarden.com/...48/5159254.htm