Why must the divination crystal ball be transparent?

First of all, the crystal ball has the function of absorbing energy, because the crystal ball itself is a mineral stone, which can emit energy and absorb energy. Maybe the fortune teller thinks this stone is amazing, and the face value of the crystal ball is very high. Because of its transparency, it has become the main divination object of fortune tellers. Moreover, because the crystal ball is transparent and can reflect light and images, it can also be used for divination, which can make the diviner more confident and mysterious psychologically. If ordinary stones have no reflection and cannot be imaged, fortune tellers don't seem so convinced.