A pendulum should not be used as a crutch to help us decide every which way to go in life. It should be used with the intention to truly help us grow, always seeking answers that can contribute to the greater good.
If you wanted to, you could use a stone attached to a piece of string as a pendulum. The reason there are different styles and shapes is purely for preference. Simply choose the shape and material that appeals to you. For a crystal pendulum you may choose a stone that has qualities that you wish to see appearing more in your life.
To find out more read: Pendulum Dowsing – An Introduction to Using a Pendulum