I heard another guy mention lava rock, that is my favorite for making rock formations really cheap. Cement mixed with cheap playsand, I found a place that sells red volcanic sand, that is awesome. Cool trick for caves in the cement is use plastic wrap and wrap it around a paper towel tube. Then just stick it in anywhere after it is partially hardened but somewhat soft tear the edge of the paper tube and it rip right out and then pull out the plastic wrap the cement will kind of drip down inside a little because the plastic wrap kept it softer inside. Looks really cool!
I really have started to just kind of look around more at places that sell stone for landscaping, they will sometime just give me some of the smaller stones or they will cost very little. Some Marine epoxy and you can really make some neat stuff, but watch out for limestone.
125Gal SW-Reef Tank, 125Gal FW-Planted Show Tank, 75Gal FW-Planted Crab and Frog Tank, 29Gal FW-Brackish Puffer Fish and Several 10Gal and 5gal Testing Tanks.
Check Out My Podcast about Aquariums, Live Entertainment, and whatever else. Its real rough right now but getting better, still funnyAquatics Live Podcast