Because of the cost of rock crystal balls many people use a glass ball but this is not advisable.
Brazil has the best crystal rocks in the world.
Although crystals may grow to a considerable length, they seldom exceed one inch in thickness. There are however some exceptions. Flaws in a crystal such as cracks, bubbles and discolorations do not make crystal unfit for scrying although they may distract your attention. Size is however not important and bigger does not mean it will be better. Crystal scrying should be done in near or total darkness.