The art to making a good magical item includes selecting the most appropriate materials... at least, it does if you are playing the Karma Roleplaying System. But even if that is not your ruleset of choice, incorporating such concepts into your game enhances the flavour of magical craftsmanship, even if it does not contribute in terms of game mechanics.
The basic idea is that each metal has a certain affinity with one or more of the elements (the classical air, earth, fire and water, or the extended series used in Karma Roleplaying System magic). Once you have decided what your magic item is going to do, which spells you will embue it with, you can then decide which metal will be appropriate. Of course, there are other considerations such as cost, weight and will it hold an edge (if you are making a magical weapon), but even if the best metal to match the magic isn't the best one for the item you have in mind, perhaps it can be used as decoration.
Some metals, the base metals, are easy to get hold of and not too expensive... however they can still be very useful to the mage seeking to make something. Noble or precious metals are also popular, although they are rarer and generally cost more than the base metals.
Whether you are using the Karma Roleplaying System or something else, the information herein should enhance the flavour of magical item creation - characters can select the metals best suited to what they want to craft (giving scope for adventure to source it of course) and items in treasure hoards can be given addition interest - "This bowl is made of platinum, perhaps it has healing properties" for example.