I have used river rock from landscape and quarry suppliers. River rock refers to rounded smooth chunks of rock that you can get in many sizes from pebbles on up to larger pieces. It is worn smooth by water action, hence the name. This should be safe to use from the perspective of toxic substances, just give it a very good wash with hot water (not boiling which can cause rock to explode) and a stiff brush.
Another aspect of rock is whether it is calcareous or not. Calcareous rock contains calcium which slowly dissolves, raising the GH and pH in an aquarium. This is fine if you intend an aquarium of hard water fish like livebearers or rift lake cichlids. But for soft water fish (angelfish, tetras, rasbora, etc) this is something you do not want. River rock is not likely to be calcareous, as it will usually be granite in composition. You can test for this with an acid like vinegar, though this is a very weak acid, and a better one would be the Regent #2 of the API nitrate test kit. A couple drops on a dry rock is all you do; if it fizzes it is calcareous.
If you are intending angelfish and other soft water fish like some of the tetra that will work with angelfish, wood should be the prime aquascaping material. The waters that these fish come from are full of sunken wood, whether roots, branches, logs. Rocks are rarely if ever seen, but that doesn't mean you can't have some, depending upon how authentic you want the aquascape. Wood has other benefits too, as it releases tannins and organics that suit such fish.
Byron.
