Iridium is one of the least abundant elements in the Earth's crust, having an average mass fraction of 0.001 ppm in crustal rock; gold is 40 times more abundant, platinum is 10 times more abundant, and silver and mercury are 80 times more abundant. Tellurium is about as abundant as iridium, and only three naturally occurring stable elements are less abundant: rhenium, ruthenium, and rhodium, iridium being 10 times more abundant than the last two. In contrast to its low abundance in crustal rock, iridium is relatively common in meteorites, with concentrations of 0.5 ppm or more. It is thought that the overall concentration of iridium on Earth is much higher than what is observed in crustal rocks, but because of the density and siderophilic ("iron-loving") character of iridium, it descended below the crust and into the Earth's core when the planet was still molten.
Iridium is found in nature as an uncombined element or in natural alloys; especially the iridium–osmium alloys, osmiridium (osmium rich), and iridiosmium (iridium rich). In the nickel and copper deposits the platinum group metals occur as sulfides (i.e. (Pt,Pd)S), tellurides (i.e. PtBiTe), antimonides (PdSb), and arsenides (i.e. PtAs 2). In all of these compounds platinum is exchanged by a small amount of iridium and osmium. As with all of the platinum group metals, iridium can be found naturally in alloys with raw nickel or raw copper. Within the Earth's crust, iridium is found at highest concentrations in three types of geologic structure: igneous deposits (crustal intrusions from below), impact craters, and deposits reworked from one of the former structures. The largest known primary reserves are in the Bushveld igneous complex in South Africa, though the large copper–nickel deposits near Norilsk in Russia, and the Sudbury Basin in Canada are also significant sources of iridium. Smaller reserves are found in the United States. Iridium is also found in secondary deposits, combined with platinum and other platinum group metals in alluvial deposits. The alluvial deposits used by pre-Columbian people in the Chocó Department of Colombia are still a source for platinum-group metals.
In the following video Professor Martyn Poliakof mentions the amount of rock that had to be dug up from deep underground, most likely in South Africa, to make the Iridium rod he was holding. I did the math to break it down into a more manageable number. It works out that 5,000 tons of rock need to be processed for each troy ounce of pure Iridium. Compare this to the average of 50 tons of rock to produce one troy ounce of Gold. Meaning that coming out of the ground Iridium is 100 times rarer than Gold.