The Rainbow Mines
One day, a peasant was walking around and saw something purple sticking out of the ground. This (naturally) confused him, and he dug the thingamajig out. It turned out, it was a purple metal. He took it to his king, who ordered a total excavation of the site.
The king decided that only his elite magic knights could use the stuff, because there wasn't that much of it. This angered the king's advisor.