JARblue

Just a few comments from The Google:

Iridium melts at a very high temperature and does not oxidize. Iridium plugs are used in aircraft piston engines for this reason.

Platinum does not melt at a high temperature. It does, however, not oxidize. SO they are good for OEM engines for people that don't want to replace their plugs all the time. Don't use them in a high-power engine, though.

Copper melts at a decent temperature and is the most thermally and electrically conductive common metals. It does oxidize, though, so they have to be changed periodically.

Iridium plugs are often marketed that the fine center electrode reduces the voltage requirement for spark.
Translation: The pinprick of pointy metal aka iridium electrode makes an excellent hot spot in the combustion chamber, allowing pre-ignition to destroy your engine. Also, lowering the voltage requirement for spark to bridge the plug's air gap reduces the amount of energy discharged from the coil, which in turn reduces the amount of amperage available to flow across the initial ion trail and form a decent plasma kernel, whereby you have trouble igniting charge densities that are mundane to a properly designed spark plug with copper, steel, or silver/nickel electrode that cost as much as 1/6th the price.

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