How abundant is tanzanite?

Tanzanite is one of the rarest gemstones in the world. It is only found in a small area in the Merelani Hills of northern Tanzania. The only known source of tanzanite is approximately 4 square kilometres (1.5 sq mi) in size. In addition to its rarity, tanzanite is also quite hard, making it difficult to mine and cut. As a result, tanzanite is a relatively expensive gemstone.