This rare gem is a variety of zoisite that is primarily mined in the Merelani Hills of the Manyara Region in Northern Tanzania. When the stone is mined, the material is brown in color. Heat treating causes the colors to change to an intense purplish blue. Tanzanite is photo dichroic, its color appears different in different types of light. These stones are more intense blue in daylight and more purple in incandescent light.