What most naturally follows?

How to prepare fish tank gravel
Select the type of gravel to put in your fish tank at your local pet or aquarium supply store.
Fish tank gravel comes in several varieties, such as glass, hard gel or stone, and may be natural or dyed. For the best results, look for natural gravel or varieties with a polymer seal so it is not likely to alter your tank's chemistry and harm your fish or other aquatic life.
Be sure that the fish tank gravel you chose is safe for fish tanks. It should indicate this on the packaging and say that it is pre-washed.