What is tanzanite and where is tanzanite found?

We put this question to Ian Mercer:

The short answer is yes, that is true. Tanzanite is a really beautiful gemstone. It's a gem variety called zoisite which really occurs as a sort of powdery crust - greeny blue and brownish colours. Certainly not gem quality. That place in Tanzania is the only place in the world where quantities of gem quality zoisite form. Chris: Chemically what is it? Ian: It's another of these silicates. It's a pretty complex silicate in fact. We won't go into the formula because it would take longer than a minute. It's an orthorhombic silicate. One of its attributes is that it changes colour as you turn the stone around. It's what's called dichroic. It's treated in order to improve that blue colour as with most gem stones.