Granite, slate, lava rock, limestone, river rock, lace rock, texas holey rock... there are many types available so you can make the tank look how YOU want it to. See if you have a landscaping or rock cutting business near you that sells rocks, it would be much, much cheaper then the lfs, which is a total rip-off.
We have used lava rock and tufa rock with a crushed coral sand(Smoothed is a must). Not only does the Tufa and the sand buffer the water, it looks very good too!