I have been collecting things to use as decor in my tanks for 40 years. the best I have is several large peices of coal I found wshed up on the beach. they were worn smooth by the surf and had many encrusted shells. The coal came civil war blockade runners that were sunk during the civil war. A large stump of a crepe myrtle bush. A chunk of polyester polymer, It's bright white! And a hollow black walnut log.