Amethyst occurs in different tones. It graduates from a very light hue to a richer more vivid purple color. It is the ferric iron that colors the stone violet or purple, but as with all quartz, it is a silicate, SiO2 Found in Brazil, Uruguai, Sri Lanka, Canada, USSR, USA and several places in Africa. As with all quartzes it has piezoelectric properties, which means it can translate mechanical stimulation into electrical charge and vise-versa. ( More information on this on Kendra Grace's book, "Aromatherapy, Crystals and Vibrational Healing", pg 145. 
Stimulates memory, enhancing mental functions. Good for insomnia and headaches. Combine with lavender for a powerful purifying influence. In Ayurveda medicine it is considered the perfect balance between Yin and Yang.