Amethyst is known as the primary healing stone because it emits rays that are considered bio-compatible and bio-resonant when preheated. This is why amethyst is used as the main crystal for generating Far Infrared Rays.  Natural black tourmaline and ceramic tourmaline disks are the most effective in generating negative ions. This is because we bake them with ores, volcanic rocks, yellow clay, gold and red ochre, elvan and other minerals to enrich anion emission to a highly healthy level of 700-1500 ions. Bian stone is an ancient material that was used for acupuncture, moxibustion, and making various healing tools, as well as musical instruments for imperial ceremonies. Despite its widespread use in ancient times, it was lost for several thousand years before being rediscovered in modern times as the 'pumice stone' from Sibin. When heated, bian stone emits ultrasonic waves with bio-resonant properties that are believed to benefit the body according to Traditional Chinese Medicine. Red agates have been used for healing since ancient times due to their various properties. The following books provide insight into the healing properties of different crystals according to traditional and holistic medicine. The Book of Stones: Who They Are and What They Teach by Robert Simmons and Naisha Ahsian https://www.amazon.com/gp/product/1556436688   The Crystal Bible by Judy Hall https://www.amazon.com/gp/product/1582972400 Healing Crystals: The A - Z Guide to 430 Gemstones by Michael Gienger https://www.amazon.com/gp/product/1844090671