Both Merlin and John Dee used a crystal globe for scrying. 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.
The main thing is to avoid reflections on the surface of the crystal. The best method is to use the light of a candle when scrying but make sure the candle does not reflect in the ball.
Focus your gaze on the centre of the crystal not on its surface. Try looking through the crystal as if it were a mirror upon the astral world.
The first thing you may see is clouds that change color. Eventually a mist will spreadoutwards from the centre of the crystal to reveal images.
When you scry for visions sooner or later you will achieve communication from the spirits. These spirits will help you to understand what you have seen in the visions.
It may be helpful to charge your crystal ball once a month with moonlight. Place the ball in a glass bowl of natural water in the glow of a full moon.