http://www.curiousnotions.com/home/metals.asp
To the left you see a logarithmic graph comparing the physical abundances, by weight in the earth’s crust, of what many would recognize to be the six most precious or “noble” metals. Ir, Au, Rh, Pd, Pt, and Ag stand for iridium, gold, rhodium, palladium, platinum, and silver respectively. These figures are at best educated guesses, but according to most sources iridium is the clear winner. It occurs at about four-tenths of a part per billion, a ratio similar to that of a grain of salt to a Clydesdale, making it about half again as rare as gold.