Tanzanite is a blue or violet coloured version of a base type of Calcium Aluminium Silicate called Ziosite. It was first discovered in a part of Tanzanie in 1967.
Tanzanite does change colours in different orientations appearing Sapphire Blue, Violet and Burgundy depending upon the orientation of the crystals in the stone, and Tanzanite will also appear different colours in different lighting conditions. It has the property "Trichroic" which means it shows the 3 colours (Blue, Violet and Brown) at the same time.
With a Hardness of 6.5 on the Mohs Hardness Scale, it is only moderately hard and is a reddish brown in it's natural state, requiring heat treatment to bring out the special colours, ususally between 370C and 390C for 30 minutes.
Some examples of Tanzanite are shown below