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This brings up several important things to remember when putting rocks in your aquarium. First and probably most important is the fact that some rocks just aren't suited for aquariums based on their composition. Certain chemicals contained in the rocks can leach into the water and play havoc with your water parameters. Some, like calcium carbonate or magnesium carbonate, will make your water hardness go through the roof as the elements dissolve into the water. Marble, limestone, coral, dolomite, or anything similar will do just that. While it's possible to live with rocks that impact your water parameters (Amano scapes often use such stones) in the long run, it just isn't worth it. Rocks with obvious metallic seams in them should also be avoided as they can also leach harmful substances into the water. A decent (although somewhat dangerous) method of testing whether or not a rock will dissolve into your aquarium is to put a few drops of acid onto it (strong vinegar or muriatic acid; be careful!) and see if it foams or fizzes after a few minutes. If it does, it likely contains an element that will affect your water parameters.
Second, rocks come from an outdoor environment. They can carry all sorts of dirt and substances on them that may be harmful to your aquarium's ecosystem. What I do is wash them thoroughly under running water with a mild soap to remove any loose dirt and oils. Then, I put them in a pot of water and boil them for a while. This kills any living things on the rock that I may not want in the aquarium. After the rocks have cooled they should be ready to be placed into your aquarium.
Using this knowledge, and a basic understanding of geology, it's possible to find unique and interesting rocks locally for your aquarium. Best of all, they're free!