Most gems are found in several places in the world. Emeralds come from Colombia but also from Zimbabwe; there's amethyst on almost every continent; and diamonds—although associated with Africa—are mined in Russia and Australia, among other places.
Not tanzanite. The stone, which is often likened to blue sapphire but is more brilliant with violet overtones, was discovered only 40 years ago, and geologists are convinced that it occurs in only one place in the world: Africa's Rift Valley, 25 miles from the base of Tanzania's Mount Kilimanjaro, in a little place called Merelani.
Twenty minutes down a dusty unpaved road, about...