Psychics have been using crystals to augment and add focus to their readings, for hundreds of years. Crystals being from the earth, have a direct connection to the spirit world and many believe they hold immense power. The belief that humans only have 5 senses is incorrect, as we have many more such as Equilibrium: balance and body movement, Thermoception: Sensing hot and cold and some of us (psychics) have this extra sense to connect to the spirit world.
Since crystals help strengthen the connection to the spirit world, they are often used most with psychic medium readings. Some psychics may hold the crystal in one hand, where others might place them around the room. Or sometimes all though a bit clichéd, they might use a crystal ball.
Every psychic has his or her preferred method of conducting psychic readings, so not all use crystals, for example some use the tarot or runes. Since every psychic’s gift is different, so too are their methods of divining. So don’t worry if you try a new psychic, and their methods are completely different, to another you have visited.