Tanzanite is a blue or violet-blue gemstone that is a variety of the mineral zoisite. Tanzanite stone is a relatively soft stone, with a Mohs hardness of 6.5 to 7.0, which makes it susceptible to scratches and damage if not handled carefully. The value of tanzanite is determined by factors such as color, clarity, size, and cut. The most valuable tanzanite is a deep, rich blue with no visible inclusions, and is typically larger in size.