But they are similar. Dr. Roddy Conrad wrote some info about it several years back as a carveable stone that will harbor biobacters in aquaria and waterfall features. I've been looking for some here in Oklahoma to experiment around with in my pond but as it is mined mainly in California it isn't readily available here.
Doc Conrad built a pretty impressive waterfall with it that functioned much the same as an open air shower. Since the water flows through it as much as over it the biobacters take up residence below the surface where UV radiation can't kill them off.