Aquarium gravel comes in every color - black, hot pink, orange, yellow, fluorescent blue. The good part about this gravel is that most of it is plastic coated and therefore inert, and won't add dust or chemicals to your tank. It is also the right size so as not to get sucked into undergravel filters. The bad part is that there's nothing natural about it. A bright-colored gravel will also detract from the colors of your fish.
More natural gravel can be bought at garden supply stores. The colors may be more earthy and the sizes of the pieces will not be uniform. You can also get gravel from a local stream. Beyond its decorative function, gravel is there to serve as a substrate to hold waste and detritus (and with an undergravel filter to serve as a bed for biological filtration) and to hold plants, if you have them. All that you should be concerned about is that the gravel is fairly uniform, clean and will not change the chemistry of your water.
Wash Gravel Before Using
Wash all gravels before putting them into the tank. Even gravel from the aquarium shop will have a layer of dust on it so put it into a clean colander and rinse it until the water runs out clear. If you have taken gravel from a stream or quarry, boil it first to make certain no unwanted plant or animal life is holed up in it.
Don't use broken shell or shell sand or any limestone that will release carbonate into the water. This will play havoc with the pH of your tank.
If you plan to put plants into the tank, make sure that the pieces are small enough to hold the plant roots. The gravel in the aquarium shop is a good size for comparison.
If you plan to keep fish that scour the bottom of the tank for food, try to get gravel without sharp edges that might injure the fish.
When setting up the tank, put in the undergravel filter if you're using one, then the gravel. Add the water slowly at first so as not too disturb the gravel. Once the tank is about half filled, re-contour the gravel, set in the plants, and then finish filling the tank.