When used in metaphysical sense
When crystals are used in the metaphysical sense, the effects on mental energy are astounding as are the effects on physical energy.
We are able to amplify our body's energy and thoughts, therefore gain greater clarity in our thinking and vision, and also more effectively influence matter.
Transformation
Try taking a glass of water and place it on a large cluster (at least 20 cm in diameter) and another glass of water about 4 meters away from it. Wait 12 hours and taste the water and see if you notice a difference in taste. (I always have rose quartz in my drinking water in the fridge)
Although there is no test to prove this, but crystals can be charged with energy for healing, teaching, pepping up ones energy levels as well as relaxation.
Crystal energy
One can also charge the crystal with ones own energy, and then allow the crystal to give it back to you when needed.
Take the charged crystal into a darkened room and place it into your left hand (palm upwards). Close your eyes and meditate on the crystal. Mentally picture the crystal, and picture its energy as white light, flowing from the crystal through your left arm into your chest filling up your whole body.
Crystal healing
Refer to my crystal layout charts in the healing section, as well as my disclaimer.
If one is at work and has a headache, one can easily overcome it by going through the crystal energy process mentioned above; only imagine the headache as a ball of black light and as the white light flows into you, it pushes the black light steadily down through your body out of your right hand and feet into the earth. Keep doing this until all the negative energy is gone.
Transference
Your crystals can be used for sending and receiving thoughts across kilometres between subjects. Your crystal becomes a corridor between mind and body. The crystal enables us to communicate with all life, even our animals, and on a spiritual plane we can use it to help us communicate with our Higher Self.
In meditation crystals can aid by helping us move into altered states of consciousness, serving as a tunnel for tapping talents and abilities. Our intuition and psychic abilities will be awakened.
If you decide to start working with crystals as tools for growth, find a crystal or crystals that resonate with your vibratory patterns unique to you.