What is iridium?
Iridium is a silvery metal of the platinum group, notable for its hardness, resistance to corrosion, and rarity. Naturally-occurring iridium alloys were used for tipping gold dip pen nibs and nibs for older fountain pens. These alloys were unrefined, and could contain as much osmium and ruthenium as iridium.
From the 'teens on, penmakers began experimenting with different tipping alloys, some of which contained little or no iridium at all. These materials were still not fully refined, so nibs predating the 1940s often display considerable variation in the the hardness and homogeneity of their tipping material.
It appears that no modern nibs use tipping material containing iridium, even though some are explicitly marked "Iridium Tipped". Regardless of the material's actual composition, "iridium" has become and remains the standard term for the hard material used to tip a nib.
Nibs that have lost their tipping material can be retipped, though for more common nibs it will be considerably cheaper to obtain a complete replacement. A nib that has lost its tipping material may still be usable. Gold is a relatively soft material, but 14K gold is still hard enough to stand up to considerable wear. This is especially the case if the tip is fairly broad and the writer is not heavy-handed.
For more, see these articles by John Mottishaw