Amethyst is the purple variety of quartz and generally is the most prized variety of quartz. When heat treated it can have a deeper color or be transformed into Citrine. Some varieties can also be changed into Prasiolite. A few common varieties of amethyst include Ametrine, Canadian Amethyst and Veracruz Amethyst. It is also the wedding anniversary gemstone for the 4th and 6th year of marriage.