Ductility
Ductility can be quantified by the fracture strain formula_1, which is the engineering strain at which a test specimen fractures during a uniaxial tensile test. Another commonly used measure is the reduction of area at fracture formula_2. The ductility of steel varies depending on the alloying constituents. Increasing the levels of carbon decreases ductility. Many plastics and amorphous solids, such as Play-Doh, are also malleable.
The most ductile metal is platinum and the most malleable metal is gold.
Nib (pen)
According to Mathur et al., "the modern fountain pen nib may be traced back to the original gold nib which had a tiny fragment of ruby attached to form the wear-point." Following the discovery of the platinum group of metals which include ruthenium, osmium and iridium, "a small quantity of iridium was isolated and used on the iridium-tipped gold dip pen nibs of the 1830s". Today, nibs are usually made of stainless steel or gold alloys, with the most popular gold content being 14 carat (58⅓%) and 18 carat (75%). Titanium is a less common metal used for making nibs. Gold is considered the optimum metal for its flexibility and its resistance to corrosion, although gold's corrosion resistance is less of an issue than in the past because of better stainless steel alloys and less corrosive inks.