Tanzanite

Tanzanite is an extremely rare gemstone and found in only one place on earth, near Mt. Kilimanjaro in Tanzania and is a relatively recent discovery. This blue variety of zoisite was named for Tanzania, the country where it was discovered, by Tiffany & Co.
Tanzanite crystals show different colors depending on viewing direction, so cutters are able to determine whether the stone will be pure blue or a blue with a purple hue depending on how it is cut.

Tanzanite is the Birthstone for December

MINERAL: Zoisite
REFRACTIVE INDEX: 1.691 to 1.700
SPECIFIC GRAVITY: 3.35
MOHS HARDNESS: 6 to 7