Although you can read every crystal book on the market, crystals (like humans) are unique and they "speak" when you listen. A crystal will vibrate, pulse, or change temperature when they can be of use. It takes practice and learning to connect with the life force of the crystal and allowing it to guide you. There are generic crystals well known for certain chakras or health concerns, but to be truly effective, you have to learn to listen to your own intuition and the crystal itself without second guessing yourself. There are also classes that teach you how to connect to crystal spirits or elemental spirits.