Stones and crystals are made by the compression and changing of the many layers of earth. Because of this, they hold powerful energies that can be used in magick.
Have you ever picked up a quartz crystal and have goosbumps pop up all over your skin? Did you feel a stinging, or tickling sensation when you touch it that travels up your fingertips and arms, until it gradually fades away at your elbow? If you are unfamiliar with the ways of witchcraft, and the mystical powers that the earth holds, you probably would have considered it coincidence, or that you caught a draft of cool air. Chances are, you just happened to come across a crystal that had high levels of magickal energies within it. This could be caused by it's development and transformation, or it could be because the previous owner of the stone had used it in her own magickal practices.
Because of this, when you do decide to buy a stone for magickal use, it is good to cleanse it. You cleanse stones the same ways you would cleanse a candle that you bought at the store. Direct sunlight will do the trick, and it is also the easiest method. Set your stone(s) out on the windowsill in the direct sunlight, and let the sun do the work for you.
Crystals are especially good in increasing the potency of the power you put into your spells. Let me give you an example: I was casting an herbal spell for a loved one of mine. I set the bowl where I would be mixing my herbs in the center of my altar. Around this bowl, I placed many types of crystals and stones that offered support in my wishes. As I placed the herbs, one at a time, in the bowl, and stirred them with my finger, I could feel my energies projecting from the stones, and soaking into the herbs with much more strength than would have been originally sent out.