It's just a type of rock. See http://www.hbhnet.com/rock.html for a picture. It's often times used as a dry base rock in aquarium since live rock can be quite expensive. I have 30 lb of lace rock in my tank (bought at a lfs for $1 / lb) and 45lb of live rock. People say that coraline algae will cover lace rock given enough time (a few months, I am not there yet)