== Occurence == 

 "Iridium is one of the rarest non-radioactive, non-noble gas elements in the Earth's crust,". Iridium is a gas? Oh, it is not as rare as some of the non-noble-gas elements. Anyway this sentence is confusing. How about: "There are few elements that are more rare than iridium. They are e1, e2, e3, etc". Or something like that. c.pergiel