Even though it is popular to depict a psychic looking into a crystal ball and mysteriously waving hands over it, a crystal ball is not one of the most often used psychic tools.
A crystal ball performs a similar function as a sensory deprivation chamber. Staring into a candle flame or a freshly painted wall, or maybe taking a rest in a coffin or a fallout shelter also operate on the same principle. (For the really adventurous, try flying up to outer space on a spaceship.)
Whether you use mental focus or physical tools, the goal is to get into a state of mind without outside distractions. When using a crystal ball, it's not about seeing something in the ball itself, but rather using it as a kind of visual 'white noise' to cancel out the unnecessary chatter of our minds. It's a shortcut to getting into a psychic trance.
What happens when you're deprived of your senses? Well, your fears and inner demons come out to greet you, and if you don't find a way to overcome them or get out of that state, you'll go mad.
If you don't let such things bother you, well, then anything can happen.