Substrates are the most overlooked part of the aquarium environment. Most new hobbyists will spend more time researching the cost and appearance of their aquarium lighting than the substrate that their livestock will be living in.
I think many spend a bunch of time of substrate since it’s something that you will always see, it really can affect the species you can have, and it can set the kind of theme or style one would decorate with.
The most common is just dyed gravel. You can see this in all pet stores. I am not sure what the proper name of it is, as to what it is made of. I prefer pebble and smooth stone natural substrate myself. They come in all sizes and they don't contain dyes or sealants.