Correct, or at least most of it. There are a few, like cinders, that are actual rock, but most of it is crushed walnut shells. The advantage of using the walnut shells is they they absorb dye easily so WS is able to make many different colors of ballast.
If you're thinking walnut shells would be easier than rock to make on your own, think again. Even if you have access to empty walnut shells, they are almost as tough as rock and you have to screen it several times during the process to remove the debris, including bugs. WS uses huge machines with a couple of hundred knife blades to run the walnut shells through, so the end product is relatively even in size. It's then screened and steam sterilized. You also need special colorfast dyes to color the walnut shells. Believe me, forking over the eight bucks and having ballast ready to go is lots easier.
If you're thinking walnut shells would be easier than rock to make on your own, think again. Even if you have access to empty walnut shells, they are almost as tough as rock and you have to screen it several times during the process to remove the debris, including bugs. WS uses huge machines with a couple of hundred knife blades to run the walnut shells through, so the end product is relatively even in size. It's then screened and steam sterilized. You also need special colorfast dyes to color the walnut shells. Believe me, forking over the eight bucks and having ballast ready to go is lots easier.