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.
No modern nibs appear to use tipping material that contains 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 may be 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