This is a video about how I make DIY rocks for my aquarium. Basically you use three ingredients: 3 parts crushed oyster shells, 1 part white portland cement #1-2, and 2 parts rock salt. Mix and mold. Cure the cement in cold water. Then to make it live, cycle it with a live rock from a freinds aquarium of the local fish store.

Before putting the rock in an established tank, test the pH of the water the cement has been curing in to make sure that the rock won't raise the pH of your system. The pH of the curing water should have a salinity at of very near 0 (SG of 1), and the pH should be no higher than the pH of your tank, but preferrably lower. (less info)