The metal trim pieces on a pen are typically made of brass. Such pieces include the nib at the tip of the pen, the centerband, the clip, and the finial at the top of the pen. These brass components are electro-plated with precious metals to give the beautiful luster that is associated with fine writing instruments. Each type of metal plating has its own physical properties, such as hardness and brittleness. This will determine how well the parts can cope with the abrasions and chemical exposure of daily use.
All platings will wear with use. For them to wear down to the point that the underlying brass begins to show will usually take several years with the softest metals. For the hardest metals, such as rhodium, it may take one or more lifetimes. The way that you care for your pen can effect this wear rate dramatically. If you avoid abrasive situations, such as placing the pen in your pocket or purse against other hard objects, and if you periodically wipe your metal surfaces clean with a damp cloth, you can significantly extend the life of your metal plating. Below is a summary of the various metal platings that are used. I've attempted to list them in order of their expected longevity (shortest to longest). Since the specific process that a manufacturer uses when electro-plating can effect its wear resistance, this ordering is merely an approximation. Each of these platings may only be available in selected pen styles.
All of the last five metals on this chart are considered to be "premium" platings will last a very long time. Several pen styles are only available in standard gold. Although standard gold plating is not as tolerant of abuse as the premium platings, I only purchase components from vendors with a good track record on their gold finishes. With proper care, your standard gold pen should maintain its appearance for several years of heavy use. Pen components can usually be replaced for a nominal charge if you want to "refresh" your pen in the future.
There are several other less common platings, such as copper, satin nickel, satin pearl, black chrome, gunmetal, and black enamel. If you are going for a specific look in your pen, let me know and I can steer you toward styles that come in these more unusual platings. These can look dramatic with the right body.