Genghis Khan and rhodium

Today I learned that Genghis Khan et al. killed 40 million people, which comes out to about a tenth of the world's population. I say 'about' because the world population dropped from an estimated 450 million people before the plague (1348-1350) to 350 or 375 million afterward.

In other news, rhodium is the world's rarest precious metal, and if you're a terribly smart investor, you can buy a little vial of rhodium powder to sell to your friends, or you could name it 'the precious,' and clutch it to your chest, preferably while muttering something moderately unintelligible.