Metal pens and pen overlays are most commonly gold filled. Generally less common and in higher demand are pens in silver, even though they originally cost less than their gold filled equivalents. Solid gold pens were the most expensive originally and are the least common today; most are 14K, but 18K was normal for France and 9K was distinctively English. 9K gold is in fact one of the most suitable material for a metal pen: it is hard and wear-resistant, will not brass, and since its gold content is low, so is its cost of manufacture. Silver filled pens and pencils are not as common as gold filled, and many collectors have wondered why such a material was used, given the relatively low cost of solid sterling. The reason would seem to be that it was much harder and resistant to marring than sterling, which easily picks up dings and dents in normal usage.
With most early metal-covered pens, the value of the metal itself contributes relatively little to the value of the pen. For later pens in solid gold, the strongest demand is from collector-users; condition, therefore, is of top concern.