Note: Platinum, Palladium, and Aluminum all have resource bases in excess of 100 years of production, and as such they are not displayed.
If the U.S Geological Data can be trusted, then silver is the rarest of all the metals left in the world, in relative terms, with a maximum of 29 years left of production using current resource numbers. Many argue that a dramatic increase in the price of silver will soon be leveled with the ramping up of production. There are several problems with this argument, especially in light of the above data.
http://www.silverinscripture.com/articles.php?id=30
Newly Mined silver must replenish supply, but a geological problem stands in the way: Epithermal deposition or condensation near the earth's surface. Simply put, the richest silver deposits are nearest the surface of the earth. The deeper mines go, the less silver they tend to produce. The deeper the mine, the more expensive the silver."