tanzanite
Tanzanite, the December birthstone, is the name given to blue to violet zoisite. It is also known for its strong pleochroism, which means that its color changes depending on the angle from which it is viewed. Its Mohs hardness is 6-7, but it has a cleavage that makes it vulnerable to impacts from certain directions, so if you wear it as jewelry, you should be careful not to expose it to strong impacts.
Carats Gem Magazine
" Tanzanite "