Or you can select your own stones one by one as it feels right to you. They should also be relatively flat, or with a flat side. You can choose your stones by virtue of their magical associations, color, name, or feel. For instance, Tiger Eye is usually included in a set of casting stones. It also could be associated with cats, vision, and the night, depending on the question and what you choose to associate it with, or what your subconscious tells you it means.
Amethyst is associated with spirituality and psychic vision, as are all purple-colored stones.
There are a number of decent books out there on the associations of stones, so getting one of those is a must if you want to do stone casting. You can cast your stones in various ways. If I'm recalling correctly, he had a circle design on his cloth, divided into four areas representing different things. As an astrologer, I invented my own method of casting the stones, not being satisfied with the directions included in the kit I purchased. Each stone has an association with a planetary energy.