The Yogo Gulch Sapphire

Yogo Gulch in the Little Belt Mountains of central Montana is one of only two places on earth that produce a naturally occurring cornflower blue sapphire of gem quality. Yogo sapphires were first discovered in 1895 when a miner found a package of what he thought were worthless blue pebbles and mailed them to Tiffany and Company in New York to be identified. The jeweler immediately recognized them as some of the finest sapphires ever seen. Yogo sapphires require no heat treatment to achieve their color, which is extremely rare among gemstones.

Source: Wikipedia