This one is a composite bubble rock.  By that I mean various rocks cemented together. We had a fairly sterile look going with this water feature we adapted and cut down in height.  By placing the bubble rock inside it, it gave a naturalistic edge to the scene plus the rock with water running looks excellent with lights hitting it at night.  Plenty of angles and shiny moments. The goal was always to drench the rock and allow the night lighting to play on it.