Because of their crystalline structure, they are able to absorb, focus and transmit subtle electromagnetic energy. This is the energy used in crystal healing/gem therapy. Sarah Seger
Gemstones have fascinated humans throughout our history for their magical and practical purposes. They were used by the Ancient Greeks and Egyptians, who understood the power of crystals in healing and spiritual development. They were said to bring luck, healing or protection to the wearer. In India, gem stones have always been used in ayurvedic medicine because of their curative powers. In Christianity, the breast plate of the High Priest contained 12 gemstones (referred to as the 12 stones of the children of Israel). Gems were also regarded as objects of great healing and power by native tribes of the Americas.
In Mediaeval Europe, gemstones were used in alchemy and astrology. In China, Japan and in New Zealand, many gems, particularly jade, were used in spiritual rites.
Science has shown that if a crystal is placed in an energy field, it collects that energy and contains it. Some crystals might change and transmit the energy during this process. Quartz usually amplifies the energy and then radiates it out again. The exception to the general principle is the black crystal which absorbs energy but do not release it. Crystal Healing for Beginners
On a practical level, crystals are able to accumulate piezoelectricity. This is a process by which material objects generate electric potential.
Quartz crystals amplify, transform, store, focus and transfer energy easily. When quartz is squeezed, it generates electricity. If a current is passed through it (AC), it shrinks and swells (oscillates), and if (DC) current is sent through it, it swells.
This osculation changes electricity into waves and can be broadcast via radio and television signals. It is also used in watches and to transmit information in computers. Quartz also focuses energy and is used in lasers, which can be used in eye surgery and also to burn through steel.
As a generator and accumulator of energy, crystals are great for energy healing, where you want to diffuse negative energy in the body or to infuse positive energy.