Here's what mine looks like. It was mixed with some old aquarium gravel that had been in the tank for at least 15 years.
You can notice on some of the 'black' stones, the black color is wearing off. That was one of the reasons I wanted to try a real gravel, not an aquarium gravel. Not sure what is used to color aquarium gravel, but considering the amount of time, it held up fairly well, but not like the all natural pea gravel. I know it's not coated with anything to give it color, so it should last forever.